DB 201

[trooper890]

Airlines and FBO's using 450-470 MHz? I've never heard of or read about air operations in this frequency range. I can see why the DB-201 would be popular for the well known airband frequencies (118-137) to direct the RF energy upward, but this model was also discontinued.

Just because you’ve never heard about it and have never read about it doesn’t mean that it isn’t happening all day everyday.

Ground ops has nothing to do with VHF air band and never has. Ever thought about the radios that the gate agent, ramp and mx is using? All uhf bud.

 

[trooper890]

No VHF or UHF aircraft base antenna I know of is designed to have an upward gain..... An aircraft overhead at 35,000ft is only 6.6mi away and absolute line of sight, you don't need any upward gain for that.

Exactly. Upward gain theory is bogus. 201’s are omni’s We’re not talking satcom here.

 

[KA0XR]

Thanks for the responses. I don't pretend to know everything about local frequency usage, but I do my best and maybe someday I'll be a true guru on the subject.

I live in a major metropolitan area not far from a huge airport but have never heard transmissions between 450 and 470 that scream "air ops" through a base station, at least not analog. I'll take your word for it that this antenna is still popular for this purpose, and next time I'm near an airport I'll look around for UHF 201's. Locally Delta uses 129.30 and 129.55 AM for a lot of air ops-like chit chat.

Would the DB-201 be desirable for GMRS repeater systems due to its lightweight and simple design? Or would most designers prefer antennas with more gain?

 

[prcguy]

I would not use a DB-201 on a repeater unless I only needed a very small coverage area like a square mile or less. For a typical repeater you want as much gain as possible put into the areas you will operate.

Thanks for the responses. I don't pretend to know everything about local frequency usage, but I do my best and maybe someday I'll be a true guru on the subject.

I live in a major metropolitan area not far from a huge airport but have never heard transmissions between 450 and 470 that scream "air ops" through a base station, at least not analog. I'll take your word for it that this antenna is still popular for this purpose, and next time I'm near an airport I'll look around for UHF 201's. Locally Delta uses 129.30 and 129.55 AM for a lot of air ops-like chit chat.

Would the DB-201 be desirable for GMRS repeater systems due to its lightweight and simple design? Or would most designers prefer antennas with more gain?

 

[mass-man]

Frequency	License	Type	Tone	Alpha Tag	Description	Mode	Tag
451.78750	WQDX671	RM	051 DPL	AirTran	AirTran - Operations	FM	Business
452.76250		M		DFW AA Bags	American Airlines - Baggage Operations	FMN	Business
464.70000	WNSS734	RM	186.2 PL	ASA Ops	Atlantic Southeast Airlines (ASA) d.b.a. Delta Connection	FM	Business
460.65000	WNWU314	RM	162.2 PL	Delta ACMX	Delta - Aircraft Maintenance	FM	Business
460.87500	KNES563	RM	127.3 PL	Delta CS	Delta - Customer Service	FM	Business
460.82500	KGX542	RM	103.5 PL	Delta Ramp	Delta - Ramp	FM	Business
460.77500	WNLY232	RM	167.9 PL	Delta othr	Delta? - Aircraft Cleaning?	FM	Business
462.40000	WQFK495	RM	162.2 PL	Frontier	Frontier Airlines	FM	Business
460.80000	WNWU314	RM	156.7 PL	USAir CSvc	US Airways - Customer Service	FM	Business
464.60000	WPMS882	RM	743 DPL	USAir Ramp	US Airways - Ramp	FM	Business
460.72500	WPES238	RM	CC 3 TG 3 SL 1	United Gate	United - Gate	DMR	Business
460.72500	WPES238	RM	CC 3 TG 4 SL 2	United Ramp	United - Ramp	DMR	Business
462.51250	WQNA248	RM	203.5 PL	Virgin Ops	Virgin America	FM	Business

When I lived just a few miles from DFW airport this was pretty interesting listening when bad weather slowed things down!!!

 

[KA0XR]

Technical question about the DB-201 design, is the radiating element folded onto itself to somehow raise the impedance from 35 to 50 ohms? Or does the folding (along maybe with horizontal radials) direct more RF toward the horizon, or both? I understand in ground plane antennas the lower impedance occurs when radials are perpendicular instead of 45 degrees, but everything technical I've read about quarter wave ground plane antenna designs specify folding the radials down.

I find the DB-201 and Kreco designs cool with the folded element and horizontal radials, but are these actually not true ground plane designs per say? In real world how important is the 50 ohm impedance matching in base antennas if mobile operations obviously have the ground plane perpendicular to the radiating element? A couple months ago I took an Arrow 2 meter (GP146) ground plane and added slightly longer radials (also from Arrow) but installed them at a 90 degree angle to the radiating element, which actually resulted in a significantly better SWR across the 2m band vs. the shorter 2m radials angled at 45 degrees.

 

[rescue161]

The original DB-201 is supposed to come with a phasing harness that is made of 75 Ohm coax and is a specific (I can't remember off the top of my head if it's odd or even) electrical wavelength to bring the feedpoint to 50 Ohms. Then your 50 Ohm coax attaches to it. I can't comment on the the Kreco design.

 

[prcguy]

A DB 201 is a folded monopole ground plane and the folded vertical element is nearly identical to a simple 1/4 element in function and impedance although it might be slightly lower impedance and a little wider band width simply due to the extra metal. The portion that goes from the top down to the ground radials is just a "1/4 grounded stub" and does not affect impedance.

I know there is a coax jumper permanently attached to the DB201 and they seem to be the same length on a VHF lo, VHF hi or UHF version, so they can't be a tuned length of 75 ohm coax for matching. I believe its just an RG-213 50 ohm jumper and since a plain old 1/4 wave ground plane with radials at 90deg or sloping can be easily tuned to 50 ohms there is no reason it should not be a 50 ohm jumper.

 

[merlin]

Agree with prcguy. a major difference is the vertical element is DC grounded and will take a direct lightning strike.
The monopole you really need lightning arrestor to keep from burning duplexers or turning a radio into a fireball.
They are one of the most rugged built antennas I know of.

 

[n4dbm]

Maybe this will help clear up some myths on the DB-201...

Decibel sold these units in several "splits" but they preferred you to order the exact frequency so they could cut it for you. The splits something to the nature of this:

DB-201A - 30-33 MHz
DB-201B - 33-37 MHz
DB-201C - 37-42 MHz
DB-201D - 42-50 MHz

All of the above models have a matching transformer section of coax inside the support pipe to correct the impedance of the radiator to 50 ohms. Anything above 50 MHz did NOT have a matching section. Around the 50 MHz frequency range, a matching transformer is (barely) not needed. This can be proven by taking an old low-band model and cutting the radiator for 50.5" above the base plate and the radials to 73 inches, and you'll get a good match at 52.5 MHz (6-meter ham). This measurement is a tad short for a 1/4 wave, but will work with a decent pattern without the matching section. As you go above 50 MHz, the ratio improves and there is no need for matching.

When you ordered a "field cut" model, you still had to specify the general frequency range you wanted from the pick listed above. Even though the "uncut" model does say 30-50 MHz, they used to still ask you for the range you wanted so they could send you the appropriate model with the right matching section.

The matching section is made up of certain pieces of 50 and 75 ohm coax. The matcher is a NONSYNCHRONOUS transformer. It is not an odd wavelength of 75 ohm coax that is used to match 100 ohms down to 50, but it is rather a certain calculated length of 50 ohm coax then attached to a calculated piece of 75 ohm coax of another certain length to correct an otherwise "weird" impedance to 50 ohms.

The unmatched DB-201 at 35 MHz may have an end impedance of, say, 32 ohms while at 45 MHz the unmatched impedance might be 42 ohms. This allowed DB products to manufacture the same trombone driven element size and diameter (and base-plate) for all antennas 30-512 MHz which is nothing short of a production miracle. The only thing they had to change is the matching transformer section.

As you get above 50 MHz, the electrical characteristics of the antenna change as the properties of low-band capacitance to ground radials and other certain things will change. Again, at 50 MHz and above, you can safely eliminate the matching section altogether and get a relatively close length to a 1/4 wave radiator above the radials.

As someone else has mentioned, the direct DC folded shunt is awesome for tower top mounting as lightning will more than likely never bother the antenna itself.

Moving on to another subject, the radials. In the ARRL handbook and by your elmers of long ago, it was advised that when you build a 1/4 wave ground plane, to make the radials 5% longer than the radiator at that frequency, and then bend the radials down 30 degrees for a better 50 ohm match. Not true for the DB-201. As you may have noticed, the ground radials must be cut WAY longer than a 1/4 wave and kept perpendicular. The reason for the otherwise strangely long radials is also for impedance matching. If you follow the 5% radial rule, you will find your match to be quite different than if you follow DB's cutting chart. The radials are very much a part of the radiating system, not solely decoupling from the feed-line. Although DB does not recommend you side mount the antenna on a tower, I have done so in many cases on 146 and 220 MHz with no problems as long as it's far enough away from the tower to not affect the match.

I have a top mounted 52 MHz DB-201 at 400 feet on a repeater, a 146 MHz model on a tower for personal use, and a 220 MHz model side mounted on a large tower a 225 feet for a simplex access point, none with a matching transformer, fed direct, and they work great. The antenna is a rock solid performer IF it is cut and installed correctly. There's really nothing that can go wrong aside from violent physical damage. The lack of multiple elements and a phasing harness like on multiple dipole arrays make them virtually perfect and reliable for high powered duplex operation. There are dozens of them in my area on fire towers and pole that have been abandoned and are probably free for the taking if you just ask.

Derek/N4DBM/WRMD298

 

[KA0XR]

Maybe this will help clear up some myths on the DB-201...

Decibel sold these units in several "splits" but they preferred you to order the exact frequency so they could cut it for you. The splits something to the nature of this:

DB-201A - 30-33 MHz
DB-201B - 33-37 MHz
DB-201C - 37-42 MHz
DB-201D - 42-50 MHz

All of the above models have a matching transformer section of coax inside the support pipe to correct the impedance of the radiator to 50 ohms. Anything above 50 MHz did NOT have a matching section. Around the 50 MHz frequency range, a matching transformer is (barely) not needed. This can be proven by taking an old low-band model and cutting the radiator for 50.5" above the base plate and the radials to 73 inches, and you'll get a good match at 52.5 MHz (6-meter ham). This measurement is a tad short for a 1/4 wave, but will work with a decent pattern without the matching section. As you go above 50 MHz, the ratio improves and there is no need for matching.

When you ordered a "field cut" model, you still had to specify the general frequency range you wanted from the pick listed above. Even though the "uncut" model does say 30-50 MHz, they used to still ask you for the range you wanted so they could send you the appropriate model with the right matching section.

The matching section is made up of certain pieces of 50 and 75 ohm coax. The matcher is a NONSYNCHRONOUS transformer. It is not an odd wavelength of 75 ohm coax that is used to match 100 ohms down to 50, but it is rather a certain calculated length of 50 ohm coax then attached to a calculated piece of 75 ohm coax of another certain length to correct an otherwise "weird" impedance to 50 ohms.

The unmatched DB-201 at 35 MHz may have an end impedance of, say, 32 ohms while at 45 MHz the unmatched impedance might be 42 ohms. This allowed DB products to manufacture the same trombone driven element size and diameter (and base-plate) for all antennas 30-512 MHz which is nothing short of a production miracle. The only thing they had to change is the matching transformer section.

As you get above 50 MHz, the electrical characteristics of the antenna change as the properties of low-band capacitance to ground radials and other certain things will change. Again, at 50 MHz and above, you can safely eliminate the matching section altogether and get a relatively close length to a 1/4 wave radiator above the radials.

As someone else has mentioned, the direct DC folded shunt is awesome for tower top mounting as lightning will more than likely never bother the antenna itself.

Moving on to another subject, the radials. In the ARRL handbook and by your elmers of long ago, it was advised that when you build a 1/4 wave ground plane, to make the radials 5% longer than the radiator at that frequency, and then bend the radials down 30 degrees for a better 50 ohm match. Not true for the DB-201. As you may have noticed, the ground radials must be cut WAY longer than a 1/4 wave and kept perpendicular. The reason for the otherwise strangely long radials is also for impedance matching. If you follow the 5% radial rule, you will find your match to be quite different than if you follow DB's cutting chart. The radials are very much a part of the radiating system, not solely decoupling from the feed-line. Although DB does not recommend you side mount the antenna on a tower, I have done so in many cases on 146 and 220 MHz with no problems as long as it's far enough away from the tower to not affect the match.

I have a top mounted 52 MHz DB-201 at 400 feet on a repeater, a 146 MHz model on a tower for personal use, and a 220 MHz model side mounted on a large tower a 225 feet for a simplex access point, none with a matching transformer, fed direct, and they work great. The antenna is a rock solid performer IF it is cut and installed correctly. There's really nothing that can go wrong aside from violent physical damage. The lack of multiple elements and a phasing harness like on multiple dipole arrays make them virtually perfect and reliable for high powered duplex operation. There are dozens of them in my area on fire towers and pole that have been abandoned and are probably free for the taking if you just ask.

Derek/N4DBM/WRMD298

Thank you for providing such an informative and myth-solving explanation of the DB 201 antenna! I’ve read that it was designed for oil drilling operations where serious vibrations were a major factor.

Aside from the impedance matching, is the other reason for the WAY longer and horizontal radials to direct more RF towards the horizon as a way to compensate for the lack of gain from the quarter wave radiating element? Since you have one on a tower for 2 meters do you find/suspect the DB-201 design is somewhat comparable to higher gain designs such as the DB-224? Or would you say performance is definitely traded for durability with the DB-201 on VHF High Band? At 400 feet at the top of a tower your repeater must have some great range, assuming other things work properly. Nothing even remotely like that here on 6 meters.

I wonder why the VHF high band version was discontinued by CommScope considering how many agencies still use VHF, not to mention 118-136 MHz civil aviation where gain is not as important? The DB-201 conjures up thoughts of it being the ultimate post-apocalyptic antenna design with its durability and immunity to lightning or wind. A local city still has one cut for I believe 46 MHz on the top of their water tower where it’s been since the late 60’s, but the radios it connected to and FCC licenses are long gone. If they still made the VHF high band version I would order one cut for 2 meters, just because its design is cool and not to mention bulletproof.

As a side project, I’ve taken an Arrow GP-146 ground plane but added longer ~24” horizontal radials (sacrificed from their GP-52 6 meter antenna which I decided to cannibalize) to create my own version of the DB-201. This mod resulted in a generally lower SWR but, more significantly, a much wider bandwidth across the 2m band and then some when compared to their factory design (20” radials sloped at 45 degrees).

Stephan
KA0XR

 

[n4dbm]

Hi Stephan,
There has been some discussion that the extra-long radials do indeed help the pattern by aiming perhaps a little more signal to the horizon than otherwise. The specs on the antenna do say 2.15 dBi, which is in theory the same thing as a 1/2 wave dipole. I've never drilled down that test, but I have compared the DB-201 to other antennas in an open range field. We've always been taught that RF follows the ground plane, so I guess furthering the length of the radials will help in that area also. Someone mentioned earlier in the post about how the Kreco design has 6 and someone else's design has 8 radials. There's probably a point of diminishing returns, and DB Products felt that 4 was enough. I have substituted low band versions of the antenna with radials shorter than the seemingly ridiculous lengths, and the match does change at the intended resonant frequency. More capacitance to ground lowers the resonant frequency, and perhaps the trombone vertical radiator has such an impedance that when cut for a physical 1/4 wave, the resonant frequency appears higher than intended, thus longer radials will lower it to the proper point. That's my experience, anyway.

As far as comparing the antenna to something to a DB224, you are talking 6 dB more gain over the DB201. Although that doesn't seem like all that much, people will argue, "well, it's not the GAIN you are talking about, it's the radiation angle!" Yes, yes.. it is.. and where does gain come from? The radiation angle !! When signal is focused more toward the horizon, it's stronger at that angle than it is up above and below the antenna where, in most cases, you don't want the signal.

I have tested the VHF high band DB201 cut for 154 MHz up against a DB222 and a DB224, along with other juvenile antennas like Comets and Diamonds, to find that indeed, the DB222 does have around 3 dB of gain over the DB201, and the DB224 has 6 dB. The tests were done in an open area with no objects of any sort around for nearly a mile, with Anritsu, Aeroflex and General Dynamics equipment (NOT just your typical "well, it's an S-5 versus an S-9 on my Baofeng meter!" type of test). If you can get by with less gain and need more survivability, then I totally recommend the DB201 over anything else because it is extremely simple and has no phasing harness or joints to break. 6 dB is right on the see-saw of tipping over the edge.. Where 3 dB is a little noticeable, and 10 dB is either there or not, 6 dB is right in the middle of "it's a noticeable and good improvement, but at what cost?"

As far as CommScope ditching the DB201 for high band, it's all about sales. No one was buying them. They ditched the low-band in June of 2017. The UHF version will be gone within a few years, as soon as they finish selling them off, if they ever do, because the demand is so low. The few people needing a rugged, zero-gain antenna will find one somewhere else, like Kreco or Sirio (RM Italy).

There are a few DB201's left up on some fire towers in my area I'd love to get, but they were put up with a man basket and a crane years ago and getting them down through the window of the tower cabin is dangerous and next to impossible. I have obtained quite a few over the years, converting some 160 MHz models to 220 MHz and air-band models to 2-meters. To answer your question on the 2-meter version, I compared mine to a Diamond X-50 at my home QTH here and can see no difference. The main difference is, the DB201 will take a direct lightning strike, where the X-50 won't. That's the main reason I changed, as storms where I live are prevalent in the summer.

I found a deserted DB-201 that had been left turned upside-down and full of water and fire ants at a Duke Energy building. It was extremely difficult to take apart and clean up. Everything will come apart, including the insulator from the base, with a little heat and some patience. There's a sticky adhesive that bonds the two after you pull the nut off from the SO-239 connector (which takes a shallow 7/8" socket, BTW). You can further clean the somewhat nasty adhesive off with brake cleaner and a scraper and start over with fresh silicone. A round wire brush in a drill press will clean the SO239 threads, and hand tools to clean up the rest. Throw the individual pieces into the dish washer and run them with a cycle with your normal dishes when the wife isn't around. The citric acid and scalding water will clean it all up very well.

Here's a pic of the 6-meter DB-201 at 400 feet, along with the DB-224 I'm using for a 2M repeater. I have taped up the vertical radiator with Scotch 33 to help eliminate precipitation static, which comes from charged sleet, snow or even high-speed rain drops. It does work extremely well, and it duplexes with zero problems.

73, N4DBM/WRMD298

 

[W8UU]

Anyone know of a radio shop hoarding a NOS or gently used DB201?
Need 48-49 MHz.
I have a checkbook.

PM me if you can help.

 

[KA0XR]

Hi Stephan,
There has been some discussion that the extra-long radials do indeed help the pattern by aiming perhaps a little more signal to the horizon than otherwise. The specs on the antenna do say 2.15 dBi, which is in theory the same thing as a 1/2 wave dipole. I've never drilled down that test, but I have compared the DB-201 to other antennas in an open range field. We've always been taught that RF follows the ground plane, so I guess furthering the length of the radials will help in that area also. Someone mentioned earlier in the post about how the Kreco design has 6 and someone else's design has 8 radials. There's probably a point of diminishing returns, and DB Products felt that 4 was enough. I have substituted low band versions of the antenna with radials shorter than the seemingly ridiculous lengths, and the match does change at the intended resonant frequency. More capacitance to ground lowers the resonant frequency, and perhaps the trombone vertical radiator has such an impedance that when cut for a physical 1/4 wave, the resonant frequency appears higher than intended, thus longer radials will lower it to the proper point. That's my experience, anyway.

As far as comparing the antenna to something to a DB224, you are talking 6 dB more gain over the DB201. Although that doesn't seem like all that much, people will argue, "well, it's not the GAIN you are talking about, it's the radiation angle!" Yes, yes.. it is.. and where does gain come from? The radiation angle !! When signal is focused more toward the horizon, it's stronger at that angle than it is up above and below the antenna where, in most cases, you don't want the signal.

I have tested the VHF high band DB201 cut for 154 MHz up against a DB222 and a DB224, along with other juvenile antennas like Comets and Diamonds, to find that indeed, the DB222 does have around 3 dB of gain over the DB201, and the DB224 has 6 dB. The tests were done in an open area with no objects of any sort around for nearly a mile, with Anritsu, Aeroflex and General Dynamics equipment (NOT just your typical "well, it's an S-5 versus an S-9 on my Baofeng meter!" type of test). If you can get by with less gain and need more survivability, then I totally recommend the DB201 over anything else because it is extremely simple and has no phasing harness or joints to break. 6 dB is right on the see-saw of tipping over the edge.. Where 3 dB is a little noticeable, and 10 dB is either there or not, 6 dB is right in the middle of "it's a noticeable and good improvement, but at what cost?"

As far as CommScope ditching the DB201 for high band, it's all about sales. No one was buying them. They ditched the low-band in June of 2017. The UHF version will be gone within a few years, as soon as they finish selling them off, if they ever do, because the demand is so low. The few people needing a rugged, zero-gain antenna will find one somewhere else, like Kreco or Sirio (RM Italy).

There are a few DB201's left up on some fire towers in my area I'd love to get, but they were put up with a man basket and a crane years ago and getting them down through the window of the tower cabin is dangerous and next to impossible. I have obtained quite a few over the years, converting some 160 MHz models to 220 MHz and air-band models to 2-meters. To answer your question on the 2-meter version, I compared mine to a Diamond X-50 at my home QTH here and can see no difference. The main difference is, the DB201 will take a direct lightning strike, where the X-50 won't. That's the main reason I changed, as storms where I live are prevalent in the summer.

I found a deserted DB-201 that had been left turned upside-down and full of water and fire ants at a Duke Energy building. It was extremely difficult to take apart and clean up. Everything will come apart, including the insulator from the base, with a little heat and some patience. There's a sticky adhesive that bonds the two after you pull the nut off from the SO-239 connector (which takes a shallow 7/8" socket, BTW). You can further clean the somewhat nasty adhesive off with brake cleaner and a scraper and start over with fresh silicone. A round wire brush in a drill press will clean the SO239 threads, and hand tools to clean up the rest. Throw the individual pieces into the dish washer and run them with a cycle with your normal dishes when the wife isn't around. The citric acid and scalding water will clean it all up very well.

Here's a pic of the 6-meter DB-201 at 400 feet, along with the DB-224 I'm using for a 2M repeater. I have taped up the vertical radiator with Scotch 33 to help eliminate precipitation static, which comes from charged sleet, snow or even high-speed rain drops. It does work extremely well, and it duplexes with zero problems.

73, N4DBM/WRMD298

Hi Derek,

Thank you for providing all this great detailed information about what has apparently become a dinosaur of an antenna! I’m sure other readers of this thread learned new information from your latest post.

I like your analogies about the differences between unity, 3 dB, 6 dB and 10 dB gain, and what it translates to in real world situations. I own a DB-222, originally cut for 150-158 MHz that my Dad helped modify down to the upper part of 2 meters. We did this by adding small metal 1.5” tabs to all 4 ends of the folded dipole loops. After researching and handling this beast I can understand the hassle with the phasing harness possibly going bad, as well as mounting difficulty compared the simpler lighter 201 design.

It sounds like CommScope priced themselves out of competition, which is too bad since the DB-201 seems like the ultimate post-apocalyptic doomsday antenna with its ability to survive hurricane force winds and lightning blasts (or derechos and ice buildup where I am in the Upper Midwest). Maybe nuclear blasts as well if not right next to ground zero? Too bad they discontinued the VHF model instead of the UHF - a base station ground plane antenna for UHF seems like an egregious under-utilization of higher frequencies where gain is much easier to attain. With both a 6m DB-201 and 2m DB-224 mounted at the top of the tower, which repeater do you find generally offers greater reliable everyday local range, assuming similar power output levels? Or are there too many other variables making this an apples/oranges comparison?

I was actually at our local airport Monday and noticed numerous ground plane antennas which looked just like the much cheaper Sirio design I looked up and you mentioned, no doubt cut for the air band. I also saw three DB-201’s on a distant building which I’m sure are going nowhere anytime soon. If I ever come into possession of a 201 I’ll certainly refer to your how-to process on cleaning it up. Looks like brand new!

73,

Stephan
KA0XR

 

[n4dbm]

I don't want to get too far off the thread topic, but the DB224 on 2-meters slightly out-performs the DB201 on 6-meters from this location. I'm running 50 watts on 2-meters through 7/8 cable with 2.2 dB of line loss. 50 watts is +47 dBm and 2.2 dB of loss gives 44.8 dBm at the antenna.. Then add 6 dB of antenna gain and that's 50.8 dBm or 120 watts ERP. On 6-meters, I'm using 1/2" cable which is also has 2.2 dB of loss at 50 MHz for the length I'm using. I'm running 100 watts on 6M, so that's +50 dBm minus 2.2 dB of line loss, or +47.8 dBm of power at the antenna with zero antenna gain is around 60 watts ERP. So, right off the bat, I'm 3 dB down in transmit power on 6M versus 2M. Even though the band characteristics of 6M would lead you to believe it would reach "father," the amount of man-made QRM in the past 30 years has made 6M a fairly noisy band at tower sites and out in the field. There's about 6 dB of degradation in signal between switching a signal generator injection tee from the antenna to a dummy load (meaning the noise floor on 6M is higher than on 2M at this site).

With 1/4 wave roof-mounted whips on the vehicle, I can carry the 2M machine around 40 miles out in all directions, while the 6M machine can be carried 30 to 35 miles. I don't think adding antenna gain (which would be very difficult to do unless you had multiple DB212 dipoles CORRECTLY installed on a large tower) would help all that much. Increasing the power from 100W to 500W would make a difference, but then your talk-in from the field would still be the same, so you'd essentially have an "alligator" repeater.

Good luck on finding DB201's to restore. Keep an eye out for them. If you use the approach that you want to obtain the abandoned antenna(s) for non-profit ham radio use, folks will be much more willing to help you I've found.

 

[KA0XR]

I don't want to get too far off the thread topic, but the DB224 on 2-meters slightly out-performs the DB201 on 6-meters from this location. I'm running 50 watts on 2-meters through 7/8 cable with 2.2 dB of line loss. 50 watts is +47 dBm and 2.2 dB of loss gives 44.8 dBm at the antenna.. Then add 6 dB of antenna gain and that's 50.8 dBm or 120 watts ERP. On 6-meters, I'm using 1/2" cable which is also has 2.2 dB of loss at 50 MHz for the length I'm using. I'm running 100 watts on 6M, so that's +50 dBm minus 2.2 dB of line loss, or +47.8 dBm of power at the antenna with zero antenna gain is around 60 watts ERP. So, right off the bat, I'm 3 dB down in transmit power on 6M versus 2M. Even though the band characteristics of 6M would lead you to believe it would reach "father," the amount of man-made QRM in the past 30 years has made 6M a fairly noisy band at tower sites and out in the field. There's about 6 dB of degradation in signal between switching a signal generator injection tee from the antenna to a dummy load (meaning the noise floor on 6M is higher than on 2M at this site).

With 1/4 wave roof-mounted whips on the vehicle, I can carry the 2M machine around 40 miles out in all directions, while the 6M machine can be carried 30 to 35 miles. I don't think adding antenna gain (which would be very difficult to do unless you had multiple DB212 dipoles CORRECTLY installed on a large tower) would help all that much. Increasing the power from 100W to 500W would make a difference, but then your talk-in from the field would still be the same, so you'd essentially have an "alligator" repeater.

Good luck on finding DB201's to restore. Keep an eye out for them. If you use the approach that you want to obtain the abandoned antenna(s) for non-profit ham radio use, folks will be much more willing to help you I've found.

Derek - Great information, thanks again for taking the time to share and for describing the dBm calculations. If one could eliminate all man made noise and keep all other things equal (same transmitter output power, same antenna height, equal cable loss per frequency and receiving antenna gain) it would be interesting to compare 6m and 2m using the simple robust DB-201 design, at least in quiet rural areas with few buildings. Since most 2 meter/VHF high band repeaters likely employ 3 dB or higher gain antennas (despite the weight and phasing harness challenges), this is probably an unrealistic real world testing scenario, notwithstanding man made noise at 6m. I gather that most DB-201’s used for VHF high band were meant to cover a very specific geographic area, operated as simplex base stations or remote bases, and not necessarily repeaters.

I’ll mention non-profit ham radio if the opportunity to ask for a potentially abandoned DB-201 crosses my path.

 

[n4dbm]

Derek - Great information, thanks again for taking the time to share and for describing the dBm calculations. If one could eliminate all man made noise and keep all other things equal (same transmitter output power, same antenna height, equal cable loss per frequency and receiving antenna gain) it would be interesting to compare 6m and 2m using the simple robust DB-201 design, at least in quiet rural areas with few buildings. Since most 2 meter/VHF high band repeaters likely employ 3 dB or higher gain antennas (despite the weight and phasing harness challenges), this is probably an unrealistic real world testing scenario, notwithstanding man made noise at 6m. I gather that most DB-201’s used for VHF high band were meant to cover a very specific geographic area, operated as simplex base stations or remote bases, and not necessarily repeaters.

I’ll mention non-profit ham radio if the opportunity to ask for a potentially abandoned DB-201 crosses my path.

Sounds great. Good luck with obtaining these abandoned antennas. There are plenty of areas where a medium to low-gain antenna is very desirable for repeater coverage. Although it's not generally thought of as a problem, very high gain antennas with a narrow beamwidth can sometimes "overshoot" the signal to where the main concentration of energy is well above the desired coverage area. We have a GMRS repeater (UHF) at only 150 feet above ground level. The height is needed to get well above nearby obstructions and into the clear. Many years ago, we started with a DB420 folded dipole array (old, used) that eventually developed a phasing harness problem. The second antenna choice was the full 10 dBd Stationmaster, also with a 7 degree focus pattern. That worked well until it developed a duplex noise problem due to a faulty joint. Not having the $1000 it would cost to buy either of these new, I changed it with a 3.8 dBd gain DB404 antenna, which I believe has a 28 degree focus pattern. I figured I would use it temporarily until we could do a fund raiser and buy the real deal like we once had before. I was sure surprised at how much improvement was made close-in to the repeater, up to 5 miles out on the low-powered portables. We did lose fringe area coverage out on the far horizon that went from 30 miles to maybe 22 in the mobile. But after some thinking, we realized that 90% of the normal repeater users benefitted from a lower gain antenna. So, yes, there are times when a robust, low-gain antenna that's super reliable and dependable outweighs gain. Such a situation is the DB201 I mentioned on the 6-meter repeater. It was a "once and done" deal and I'll probably never have access to it again.

Every situation is different. But they do make great repeater antennas if they cover your desired area. We have to rethink our objective sometimes. Not every repeater in the world absolutely has to have maximum range on the far horizon. It just needs to cover the intended area.

Good luck again.
73, N4DBM.

 

[prcguy]

I was fumbling through some of my bookmarks and came across this which I had forgotten about. This guy has some good info on folded monopole ground planes and plans for making a UHF version including the 75 ohm matching section.

Home-Brew 440 MHz Folded Monopole by KC4FWC

f5ad.free.fr

 

[n4dbm]

I was fumbling through some of my bookmarks and came across this which I had forgotten about. This guy has some good info on folded monopole ground planes and plans for making a UHF version including the 75 ohm matching section.

Home-Brew 440 MHz Folded Monopole by KC4FWC

f5ad.free.fr

"That guy" was me 20 years ago I had nearly forgotten about that article. I do remember that particular antenna indeed DID have close to a 100 ohm feed point and required a 1/4 wave matcher from some TV coax. My tools back then were mediocre at best, so I wouldn't put too much faith in it.. But I do remember the antenna working well for what I needed it to do, which was get my signal out of the valley.

73, N4DBM.

 

[KA0XR]

Thanks for posting this! Interesting how-to on replicating the DB-201 design using inexpensive materials, I will have to try doing this sometime.

Back to an impedance question, for simple VHF and above (2m) quarter wave groundplanes, does impedance really not make a difference performance-wise despite what everyone (ARRL, antenna handbooks, elmers, etc.) seem to recommend with folding radials down 45 degrees? Obviously a quarter wave whip antenna on the roof or trunk of a vehicle has its ground plane 90 degrees in all directions.

I ask because I have a thing for the horizontal radial ground plane design and want to build a copper wire ground plane for 2 meters per the design in the below YouTube video where the radials are left at 90 degrees. I also have an Arrow GP146 and found OK SWR (1.5 was lowest I could get) with the supplied 20” radials angled down at 45 degrees, but when I replaced with 24.5” radials (exact same material) at a horizontal angle the SWR was much better (1.1 in the middle of the 2m band) and also found a more predictable bandwidth.