127MHz Folded Monopole Antenna For Airband

Merovingian

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I was wanting a dedicated airband antenna a while back. I was wanting something ready made so I know it is made right. I was planning to order a J-Pole antenna but I saw a reply from PRCGuy here Aviation J-Pole Antenna regarding an antenna with a vertical element that is bent down in a 180° turn. A Commscope DB201-L which would be cut for 127MHz. It seems this antenna has been discontinued so I may have to build one of my own.

I have been doing some research on this antenna to find out exact dimensions and how it is constructed. I have managed to find some Commscope documents about cut size measurements so that gives me some length dimensions but I don't have any dimensions on bend radius (spacing of the folded part) and element diameter. I haven't been able to find any good closeup photos of the actual antenna parts, just line drawings. Does anyone know anything about this antenna or have any additional info on how it is constructed.

Thanks!
 

nd5y

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Try searching for DB201 on repeater-builder.com. They are likely to have more documentation. Also try searching for Decibel Products instead of Commscope. I was never able to find exact dimentions of the base part and driven element spacing.

I tried modeling those and I know that the spacing matters and the folded element diameter has to be larger on the feedpoint side than the grounded side. The radials have to be about 1/3 wavelength instead of the normal 1/4 wavelength.
 

Merovingian

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Okay, thanks for the information, I'll do a search. Thanks for the other info, I had thought of making it all the same diameter but if the different diameters matter then I will need to reconsider that. It may be more complex than I thought.

Try searching for DB201 on repeater-builder.com. They are likely to have more documentation. Also try searching for Decibel Products instead of Commscope. I was never able to find exact dimentions of the base part and driven element spacing.

I tried modeling those and I know that the spacing matters and the folded element diameter has to be larger on the feedpoint side than the grounded side. The radials have to be about 1/3 wavelength instead of the normal 1/4 wavelength.
 

Groeteschele

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I was wanting a dedicated airband antenna a while back. I was wanting something ready made so I know it is made right. I was planning to order a J-Pole antenna but I saw a reply from PRCGuy here Aviation J-Pole Antenna regarding an antenna with a vertical element that is bent down in a 180° turn. A Commscope DB201-L which would be cut for 127MHz. It seems this antenna has been discontinued so I may have to build one of my own.

I have been doing some research on this antenna to find out exact dimensions and how it is constructed. I have managed to find some Commscope documents about cut size measurements so that gives me some length dimensions but I don't have any dimensions on bend radius (spacing of the folded part) and element diameter. I haven't been able to find any good closeup photos of the actual antenna parts, just line drawings. Does anyone know anything about this antenna or have any additional info on how it is constructed.

Thanks!
www.commscope.com/product-type/antennas/base-station-antennas-equipment/

www.changpuak.ch/electronics/Dipole_folded.php



I hoped this help.
 
Last edited:

prcguy

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I have some DB201 folded ground plane and KRECO folded coaxial antenna parts in the garage, I'll measure them today and post the dimensions. From what I'm seeing the ground plane is just typical 1/4 wavelength and the hot part of the folded vertical is thicker mostly for mechanical reasons.

I've seen an article on building a folded momopole ground plane for UHF using house wire for the folded element and the article mentioned the overall impedance was high like 100 ohms or similar and they used a length of 75 ohm coax for matching. I have never heard of the impedance being unusual and antennas from DB Products or KRECO seem to be 50 ohm. Some do come with an internal coax jumper that could be 75 ohm but that would not make sense if the antenna is tunable because the coax stub would also have to be tuned.

Another interesting thing is KRECO makes folded monopole ground planes but they also have a VHF aircraft band ground plane that is simply a 1/4 wave ground plane with a slightly fatter than usual vertical section. Check this link for the folded ground plane then the aircraft band ground plane. Kreco Antennas - Catalog

KRECO makes coaxial antennas with and without a folded top whip. There is a site that caters to 6m repeaters and a favorite 6m base antenna is the KRECO folded coaxial and they mention its got enough band width to easily cover the transmit/receive spacing where the non folded whip version does not, showing the folded monopole has a greater band width due partially due to more metal hanging out there.

Okay, thanks for the information, I'll do a search. Thanks for the other info, I had thought of making it all the same diameter but if the different diameters matter then I will need to reconsider that. It may be more complex than I thought.
 

Merovingian

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I have some DB201 folded ground plane and KRECO folded coaxial antenna parts in the garage, I'll measure them today and post the dimensions.
That would be great! Thanks! If you are able to post any pictures of the various parts that would be helpful.

Another interesting thing is KRECO makes folded monopole ground planes but they also have a VHF aircraft band ground plane that is simply a 1/4 wave ground plane with a slightly fatter than usual vertical section. Check this link for the folded ground plane then the aircraft band ground plane. Kreco Antennas - Catalog
Yes, interesting, I just looked at the link. Similar to the DB201, they look expensive. . . I found a link to someones page that still had their page up for the DB201-L, I think, they had been selling it before for $900+ before it became "no longer available".

I found a few links to the DB201 manuals and things, I haven't found much on how it is constructed at the top of one of the manual's pages it states: "The DB201 antenna features a unique molded epoxy feed through insulator. This design affords easy replacement of the connecting lead by means of a male-to-female connection which is completely protected from the weather." I would also have to proximate this as well, I'm guessing the distance between the base plate and the start of the active element is also critical. I would guess there is a solid conductor through the epoxy, between the driven element and the coax connector that it attaches to.


I have some DB201 folded ground plane and KRECO folded coaxial antenna parts in the garage, I'll measure them today and post the dimensions. From what I'm seeing the ground plane is just typical 1/4 wavelength and the hot part of the folded vertical is thicker mostly for mechanical reasons.

I've seen an article on building a folded momopole ground plane for UHF using house wire for the folded element and the article mentioned the overall impedance was high like 100 ohms or similar and they used a length of 75 ohm coax for matching. I have never heard of the impedance being unusual and antennas from DB Products or KRECO seem to be 50 ohm. Some do come with an internal coax jumper that could be 75 ohm but that would not make sense if the antenna is tunable because the coax stub would also have to be tuned.

Another interesting thing is KRECO makes folded monopole ground planes but they also have a VHF aircraft band ground plane that is simply a 1/4 wave ground plane with a slightly fatter than usual vertical section. Check this link for the folded ground plane then the aircraft band ground plane. Kreco Antennas - Catalog

KRECO makes coaxial antennas with and without a folded top whip. There is a site that caters to 6m repeaters and a favorite 6m base antenna is the KRECO folded coaxial and they mention its got enough band width to easily cover the transmit/receive spacing where the non folded whip version does not, showing the folded monopole has a greater band width due partially due to more metal hanging out there.
 

prcguy

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Here is a picture of the top end of a DB Products DB201 cut in the 40MHz range. The main radiator is about 1" dia and the return conductor is 5/16" dia with roughly 2 5/8" spacing center to center. The other picture is the base of a KRECO VHF folded monopole over coaxial skirt and its main radiator is 3/4" dia and it had a 3/16" dia return conductor that ran about 2 1/4" spacing center to center then did a 90 deg turn into a set screw in the base. The 3/4" dia main radiator was not full length on the KRECO and I would estimate it ran about 3/4 the length of the total vertical span before making the U turn at the top. I think the large diameter was partially for mechanical strength and maybe for a little band width.

For a VHF air antenna I think a center to center spacing similar to the KRECO would be fine, around 2 1/4" to 2 1/2" center to center.

1.JPG2.JPG
 

Merovingian

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Hey! Fantastic!! Thank you very much for the information and the pictures, that fills in a lot of blanks. I have a couple more questions you may or may not be able to answer.

Is the height of the insulator between the ground plane and the base of the main element very critical?

You mentioned in another post that the low frequency model can be cut to tune it for higher frequencies. I'm guessing the cut would be at the tip of the small diameter part. It looks like from your picture that the interface between the large diameter and small diameter is welded so that part cannot be changed. So my question is:

For the lower frequency lengths, did the smaller diameter part of the main element protrude well below the ground plane, before it was cut? If it only reached the ground plane where it was clamped in place, then when it was cut shorter for higher frequencies the end would no longer reach the clamping point to connect it to the ground plane.

Earlier I was looking at the cut chart for the DB201. It seems the DB201 was only a 1/4 wave when cut for 127 MHz. I was thinking of making mine at least 1/2 wave. I looked at full wave but that would be VERY tall.

Thanks again for your help and information.


Here is a picture of the top end of a DB Products DB201 cut in the 40MHz range. The main radiator is about 1" dia and the return conductor is 5/16" dia with roughly 2 5/8" spacing center to center. The other picture is the base of a KRECO VHF folded monopole over coaxial skirt and its main radiator is 3/4" dia and it had a 3/16" dia return conductor that ran about 2 1/4" spacing center to center then did a 90 deg turn into a set screw in the base. The 3/4" dia main radiator was not full length on the KRECO and I would estimate it ran about 3/4 the length of the total vertical span before making the U turn at the top. I think the large diameter was partially for mechanical strength and maybe for a little band width.

For a VHF air antenna I think a center to center spacing similar to the KRECO would be fine, around 2 1/4" to 2 1/2" center to center.

View attachment 87708View attachment 87709
 

prcguy

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The low band element had to be cut from the rest of the antenna due to corrosion and I also salvaged the ground radials but threw away the base plate. I believe the vertical element detached from the insulator and it had a clamp for the smaller dia return element, so it was tuned by cutting the bottom end. I don't remember the small dia element protruding below the antenna base that held the ground radials, I think the clamp for that mounted flush on the antenna base plate.

The height of the insulator should not matter, its an insulator and the antenna starts where the metal inside the insulator starts.

A folded monopole ground plane would be a 1/4 design only, no half or full wave. Those would need some complicated matching at the base since they would be very high impedance at the feedpoint.


Hey! Fantastic!! Thank you very much for the information and the pictures, that fills in a lot of blanks. I have a couple more questions you may or may not be able to answer.

Is the height of the insulator between the ground plane and the base of the main element very critical?

You mentioned in another post that the low frequency model can be cut to tune it for higher frequencies. I'm guessing the cut would be at the tip of the small diameter part. It looks like from your picture that the interface between the large diameter and small diameter is welded so that part cannot be changed. So my question is:

For the lower frequency lengths, did the smaller diameter part of the main element protrude well below the ground plane, before it was cut? If it only reached the ground plane where it was clamped in place, then when it was cut shorter for higher frequencies the end would no longer reach the clamping point to connect it to the ground plane.

Earlier I was looking at the cut chart for the DB201. It seems the DB201 was only a 1/4 wave when cut for 127 MHz. I was thinking of making mine at least 1/2 wave. I looked at full wave but that would be VERY tall.

Thanks again for your help and information.
 

Merovingian

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The height of the insulator should not matter, its an insulator and the antenna starts where the metal inside the insulator starts.
Okay, great. That clears that up.

A folded monopole ground plane would be a 1/4 design only, no half or full wave. Those would need some complicated matching at the base since they would be very high impedance at the feedpoint.
I see. Does the fact that it is a 1/4 wave antenna lower it's performance much compared to a 1/2 wave antenna? I had always heard that it is best to make antennas full or 1/2 wavelength when possible.

The low band element had to be cut from the rest of the antenna due to corrosion and I also salvaged the ground radials but threw away the base plate. I believe the vertical element detached from the insulator and it had a clamp for the smaller dia return element, so it was tuned by cutting the bottom end. I don't remember the small dia element protruding below the antenna base that held the ground radials, I think the clamp for that mounted flush on the antenna base plate.

The height of the insulator should not matter, its an insulator and the antenna starts where the metal inside the insulator starts.

A folded monopole ground plane would be a 1/4 design only, no half or full wave. Those would need some complicated matching at the base since they would be very high impedance at the feedpoint.
 

prcguy

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A half wave dipole or properly designed end fed half wave will slightly outperform a 1/4 wave ground plane. A half wave center fed dipole is easy to feed but an end fed will be in the several thousand ohm range and will require some kind of matching circuit. You can make a half wave end fed but be ready for some challenges to make it work.

Okay, great. That clears that up.



I see. Does the fact that it is a 1/4 wave antenna lower it's performance much compared to a 1/2 wave antenna? I had always heard that it is best to make antennas full or 1/2 wavelength when possible.
 

Merovingian

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I'm no where near advanced enough to make something like that work. I imagine I would need several pieces of test equipment as well. I was planning to get the new V2 of the NanoVNA, I've heard a lot of good things about it. I read that there is supposed to be one coming out in mid July with a 4" touchscreen so I am waiting for that. That would be my first and only piece of test equipment for antenna analysis.

I will have to stick with 1/4 wave antenna then, I will have to start researching materials to try to construct it.

Thanks once again for all of your help and information. I'll dwell on the information you have provided and come back again if I have anymore questions.

A half wave dipole or properly designed end fed half wave will slightly outperform a 1/4 wave ground plane. A half wave center fed dipole is easy to feed but an end fed will be in the several thousand ohm range and will require some kind of matching circuit. You can make a half wave end fed but be ready for some challenges to make it work.
 

prcguy

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I think you are scaring yourself away from a fairly simple project. Just use a ground plane calculator to find the length of ground radials for about 127MHz, then make the folded monopole out of #12 house wire as an experiment and make it maybe 10% longer than a calculated 1/4 wave element for 127Mhz with a 2 1/2" spacing between the folded elements. You could solder everything to an SO-239 or mount an SO-239 on some sheet metal that you can solder. Then leave the ground radials alone and trim the folded monopole until it matches at 127MHz then measure the 1.5:1 or 2:1 VSWR band width. That will tell you if its worth making one out of better materials and if it will cover the entire VHF air band to your satisfaction.


I'm no where near advanced enough to make something like that work. I imagine I would need several pieces of test equipment as well. I was planning to get the new V2 of the NanoVNA, I've heard a lot of good things about it. I read that there is supposed to be one coming out in mid July with a 4" touchscreen so I am waiting for that. That would be my first and only piece of test equipment for antenna analysis.

I will have to stick with 1/4 wave antenna then, I will have to start researching materials to try to construct it.

Thanks once again for all of your help and information. I'll dwell on the information you have provided and come back again if I have anymore questions.
 

Ubbe

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I was thinking of making mine at least 1/2 wave. I looked at full wave but that would be VERY tall.
Never think of doing a full wave antenna. It will have unsuitable direction loobs for normal radio monitoring. There's a reason to why they settle with 5/8 and 3/4 wave lenghts antennas and connect elements in series with matching devices to get more gain.

/Ubbe
 

Merovingian

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I will give it a try. I was saying I was no where near advanced enough at making antennas to be able to create and test a matching circuit to bring thousands of ohms down to 50 if I wanted to make a half wave version of the antenna. I have no test equipment yet to be able to test the performance of whatever I make. I'll have to wait until I get a NanoVNA V2 to check the performance.

I think you are scaring yourself away from a fairly simple project. Just use a ground plane calculator to find the length of ground radials for about 127MHz, then make the folded monopole out of #12 house wire as an experiment and make it maybe 10% longer than a calculated 1/4 wave element for 127Mhz with a 2 1/2" spacing between the folded elements. You could solder everything to an SO-239 or mount an SO-239 on some sheet metal that you can solder. Then leave the ground radials alone and trim the folded monopole until it matches at 127MHz then measure the 1.5:1 or 2:1 VSWR band width. That will tell you if its worth making one out of better materials and if it will cover the entire VHF air band to your satisfaction.
 

Merovingian

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I had never thought along those lines, I thought once an antenna reached a certain frequency it became impractical to make it full wave so manufacturers went to 1/2 or 1/4 to save space or cost. I still learn as I go. . .

Never think of doing a full wave antenna. It will have unsuitable direction loobs for normal radio monitoring. There's a reason to why they settle with 5/8 and 3/4 wave lenghts antennas and connect elements in series with matching devices to get more gain.

/Ubbe
 

KA0XR

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Since this thread is still open, I have a few somewhat related questions about the DB-201 antenna design that I'm hoping someone can answer. I've read this model was the original DB Products antenna from the 1950's, predating all the other higher gain DB Product offerings.

According to the design, the 1/4 wave radiating element length makes sense, but the cutting chart for the ground radial lengths are significantly longer than a quarter wave (i.e. for 146 MHz they specify the vertical element to be 18 3/8" but with 27 1/4" radials). Any idea why this antenna was designed to have radials so much longer than the vertical radiator? Do the longer ground radials change the resonance or radiation pattern, or add a slight more gain to the antenna?

Or, is it the case that the longer radials help raise what I presume would be a low (30 ohm?) impedance value? The radials flare straight out at a 90 degree angle instead of a 45 degree angle. To my understanding, the 45 degree downward radial angle is necessary to achieve 50 ohm impedance on ground plane antennas.

Obviously, DB Products and the original designer(s) are long gone. I'm curious because a local municipal water tower still has a long abandoned lowband (45 MHz) DB-201 mounted at the top, where it's likely been for 50+ years. Through binoculars, the antenna looks like it is still in fine shape, owing to the robustness of this simple design. Thanks in advance for any insight on this.
 

cmdrwill

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A "flat" ground plane antenna, IE DB201. has a higher angle for the main lobe. Like 25 to 30 degrees up.

Why the longer than 1/4 wave radials, possibly to bring down the main lobe angle.
And also about 2 inches of the radial rods on the VHF DB201 was where the small u bolts held the radials on the base plate.
 

nd5y

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Or, is it the case that the longer radials help raise what I presume would be a low (30 ohm?) impedance value? The radials flare straight out at a 90 degree angle instead of a 45 degree angle. To my understanding, the 45 degree downward radial angle is necessary to achieve 50 ohm impedance on ground plane antennas.
The radials have to be longer and horizontal for impedance matching. It's different from a normal 1/4 wave ground plane.
 
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