Exposed Dipole Omni Antenna MOD

[krazybob]

I have a DB 422 Exposed Dipole Omni Antenna -- two folded dipoles on a pole 180 degrees opposite with a phasing harness. I want to modify this from 2 meters where it is currently at. From end to end it is 37" and would need to be lengthened for the aircraft band.

I know that to take a 155MHz antenna one can drill into the loop and add a brass screw and lower it to 2 meters using 2-3" brass screws, one each on each element. But how about 2 meters to aircraft band?

How long should the screws be? What should the distance between the elements be increased to? And how about the harness?

 

[prcguy]

You would also have to modify or make a new longer phasing harness. I would just make an entire new antenna and you can use the construction plans from a prior RR project called "VHF 4-bay dipole array" or something similar and just build a 2-bay version scaled to about 127MHz.
prcguy

 

[krazybob]

I think a 1/2 wave coaxial sleeve antenna might work well too. Slight gain. Add a ground plane and add about 3dB. I'll look for the project you mentioned.

 

[jim202]

I think you will find that that antenna will work fairly well at the 2 meter frequencies with no mods. However, if your looking to go to the bottom of the 2 meter band, then yes, you would probably have to make some changes.

you might try looking on the repeater builders site for further information on the antenna you have. You might try calling it a DB-222 rather than the 422 you used. That is a 450 range antenna.

 

[krazybob]

The model IS the DB222 and I typo'd. I didn't think anyone would notice Yes, enough metal in the air will pick up anything. But using the right tool for the job is the better choice. Unfortunately commercial antennas are $3k.

There is another way. Make a collinear antenna using multiple elements of coax in sets of 4. But not too much gain. We get gain by narrowing the aperture of an antenna and 6dB narrows to the horizon; air planes fly up in the sky

 

[majoco]

Receive only?

Confucious he say "One test worth many opinions"

 

[krazybob]

I have no desire to interfere with airborne communications. But during the firestorms around here I do like a strong communications system to listen. It is almost all about the antenna.

 

[prcguy]

Here is a link to the dipole array project, you can use the basic construction ideas and scale it to VHF air band.
http://forums.radioreference.com/build-your-own-antenna/109144-4-bay-vhf-dipole-array-project.html
prcguy

 

[krazybob]

Your antenna is excellent for public safety communications. The average monitoring listener is ground-based and has a distance to their horizon of 40 miles based on the pytagerus therum.

But your antenna is not ideal for aircraft monitoring. Its gain is maximized at the horizon with a vertical beamwidth of 16 degrees. 8 up and 8 down. To visualize this take out a protractor and set its blade on a flat surface. Now lift one end up 8 degrees. Up close it is flat to the horizon and only at a distance will it show gain. I have yet to find an antenna for the aircraft band that has gain. Even the $3,000 Sinclair Technologies Online Product Catalog - SC6172(E2712) has unity gain but a whopping 75 degrees of vertical beamwidth. That's almost straight up!

Obviously aircraft are at all altitudes near and far and this is why discones and other unity gain antennas are preferred. Their patterns are like round globes. It has been said that a large amount of wire in the air will work and that applies to your version of a DB224. I own two repeaters that both use the DB224 and at 6,400 feet it covers a huge territory. But it sucks in the mountains down below it. With only 8 degrees of natural down tilt it sees out in front of the mountains. Turn that upside down and you have the same issues with aircraft.

I'll sure give you credit for an excellent and affordable antenna for public safety!

 

[nanZor]

Their patterns are like round globes.

Well, actually only to about 3-5 times the lowest design frequency. Above that, say at 800 mhz for the typical scanner-user's discone starting at 118mhz or so, the lobes are 45 degrees or higher. But most users like them at 800mhz because the isotropic gain (a misnomer really), combined with the skewed upward looking pattern at 800 lets their handhelds cope with what would have normally been overload.

Thank goodness for things like EZnec.

Kind of the opposite affect is in operation here with the high-gain at the horizon antenna for aircraft use - aircraft are much closer even at altitude, and there is no ground-clutter to attenuate the signal, so in operation, it works out very well even if you don't have a high-angle lobe.

However, if you really do want a cloverleaf pattern for aircraft monitoring, say when you are nearly ground-mounting and have a lot of nearby attenuating ground clutter, you can use a full wave dipole. I've done it and it is an interesting specialty-niche antenna.

Just cut a vertical *full-wave* dipole, and feed it a quarter-wave from one end. Attach directly to 50 to 75 ohm coax (not too critical since we are rx-only). Perfect cloverleaf pattern every time. It looks suspiciously like an ocfd off-center fed dipole, which it is physically, but being fed only a quarter-wave in, actually provides a decent match to coax with no transformer needed obviously.

Build one - maybe you'll dig it!

 

[LtDoc]

From these posts I think you can see that your antenna wouldn't be the best choice because of it's radiation/reception pattern. It 'looks' for a signal where you normally don't find aircraft, too low. It's behaving just as it was intended.
Using an antenna that 'looks' higher rather than lower would be a preferred antenna type for aircraft, then you can make it resonant on the desired frequency range you want. One such antenna is the simple 1/4 wave 'groundplane' type antenna. Mounting it higher would extend it's 'vision', make the horizon further away 'visible'. It wouldn't have the gain your antenna has, but that's not as important as people might think.
Save that DB-222 for something it's more suited for.
- 'Doc

 

[krazybob]

After working through it with you guys I have to agree that the DB222 functions very similar to the 4-bay Exposed Dipole project but at half the gain.

But I think that it's very important to note that the 4-bay Exposed Dipole Array project is excellent antenna for listening to the public safety stuff beginning at 138MHz. PRCGUY did a great service by presenting it here.

When side-mounted to a tower as intended they offer 9dBd of gain in a cardoid pattern. From ground level that is great coverage. For aircraft a unity gain is the better choice and I don't know what the heck I was thinking when I asked about modifying the DB222.

 

[prcguy]

For aircraft DXing a very high gain antenna with lobes at the horizon is very good and I don't know how many times I've said this but even with something like a 4-bay dipole array on VHF aircraft band with a huge hole above the horizon, aircraft high and out of the antenna pattern are fairly close and line of sight, so they will still come in loud and clear.

Airports are not usually not concerned with communication with aircraft outside their controlled area and do not need extreme gain at the horizon, they need broad bandwidth to cover the entire frequency range. Thats why Discones are popular for airport use and all the advertised patterns from commercial Discones I've seen show a downward pattern at the low end of their freq range that rises to slightly above within the VHF aircraft band and not a high angles within the band of interest.

A Discone used over its entire 8:1 frequency range will incur a high upward angle starting about half way through its range but even for a Discone with a 100MHz low end its still not a very high angle at 400MHz or the upper end of the UHF air band when the Discone is used for both bands.

The same antenna at 500MHz is now starting to point upwards and at 800MHz its really bad. The point I'm trying to make is aircraft antennas are not chosen for any upward pattern. I have many antennas surplussed from commercial and military airports and some are simply a very fat center fed vertical dipole with a huge upward hole in the pattern. These are far more common at airports than Discones.

So if your hobby is picking up aircraft comms at extreme distance get an antenna with lots of gain at the horizon and forget about any upward lobes, you will hear everything close and far just fine.
prcguy

From these posts I think you can see that your antenna wouldn't be the best choice because of it's radiation/reception pattern. It 'looks' for a signal where you normally don't find aircraft, too low. It's behaving just as it was intended.
Using an antenna that 'looks' higher rather than lower would be a preferred antenna type for aircraft, then you can make it resonant on the desired frequency range you want. One such antenna is the simple 1/4 wave 'groundplane' type antenna. Mounting it higher would extend it's 'vision', make the horizon further away 'visible'. It wouldn't have the gain your antenna has, but that's not as important as people might think.
Save that DB-222 for something it's more suited for.
- 'Doc

 

[krazybob]

DXing was never a question. I asked about modifying a DB222. Nonetheless, an exposed dipole array is intended to have all of it's 26 feet side-mounted on a tower. Gain antennas achieve gain by focusing their aim at the horizon. No matter how you slice it your design is not suited for effective local and mid-range communications.

 

[krazybob]

One more thing. I don't want it to go unsaid or be implied that the design and construction plans offered by PRCGUY lack technical merit. Just the opposite. He has presented the readers here with an excellent project that will yield excellent VHF/UHF performance.

My listening experience is unique. I live at 6,389 feet over-looking southern California. Because I can hear a flea hop 100 miles away I tend to use log periodic antennas aimed at zones. This avoids interference and clutter. I have at least one commercial antenna and receiver for each band from 30-900MHz. Some are on an 85 foot guyed tower including my Motorola MSF 5000 commercial repeater on 2 meters. Combiners feed individual commercial receivers based on band. The only exception is that I use a discone on the air band to feed my commercial Bendix KX-99 receiver. This is how and why when one of our famous fires breaks out I can hear so well.

But PRCGUY's antenna could also be an advantage up here. The narrow vertical beamwidth of his design would run right across the average altiude of 6,500 feet where general aviation flies. Unfortunately that leaves a lot of space above and below me that the antenna couldn't see. I'd love to afford the Sinclair with its 75 degree beamwidth and 1dBi of gain. But I an also build a collinear Stationmaster-like antenna using 1/2 hard line that would yield gain and more vertical beamwidth.

Nonetheless, PRCGUY's antenna is well suited for public safety work. He has done an admirable job putting the pieces and documentation together.

My home and monitoring location:

 

[prcguy]

Thank you for the flowers on the 4-bay dipole array project. It started out with a RR thread asking about how to build a good antenna for VHF and I promised the person I would post some plans but it took me months to finally get some time to start the project.

After doing some research and gathering hardware the prototype was built in one afternoon and documented as it was built. Writing the instructions and cropping pictures took maybe a few hours and now we have a repeatable antenna project. I wish all projects went that smooth and quick but I got lucky on that one. I'm not even sure who the person was I wrote it for or if they ever built one.

Anyway, I'm convinced if you had a fairly high gain VHF air band antenna like a 4-bay dipole with its 16deg 3dB beamwidth, you would find it perks up those extreme distance aircraft at or over the horizon and you will not miss any aircraft at any unobstructed range or angle even though they would be in the worst null of the antenna.

All of my handheld scanners pick up aircraft and just fine at several mi out with no antenna connected at all, which is probably a much worse case than being in a 20 or 30dB null of a 6dBd gain antenna that's mounted outside and in the clear.

And for Bob not wanting to spend big $$ for a new Sinclair aircraft antenna, I have a new Antenna Products DPV-35 antenna that's pretty standard these days for airports and FAA sites that I might part with.
prcguy

One more thing. I don't want it to go unsaid or be implied that the design and construction plans offered by PRCGUY lack technical merit. Just the opposite. He has presented the readers here with an excellent project that will yield excellent VHF/UHF performance.

My listening experience is unique. I live at 6,389 feet over-looking southern California. Because I can hear a flea hop 100 miles away I tend to use log periodic antennas aimed at zones. This avoids interference and clutter. I have at least one commercial antenna and receiver for each band from 30-900MHz. Some are on an 85 foot guyed tower including my Motorola MSF 5000 commercial repeater on 2 meters. Combiners feed individual commercial receivers based on band. The only exception is that I use a discone on the air band to feed my commercial Bendix KX-99 receiver. This is how and why when one of our famous fires breaks out I can hear so well.

But PRCGUY's antenna could also be an advantage up here. The narrow vertical beamwidth of his design would run right across the average altiude of 6,500 feet where general aviation flies. Unfortunately that leaves a lot of space above and below me that the antenna couldn't see. I'd love to afford the Sinclair with its 75 degree beamwidth and 1dBi of gain. But I an also build a collinear Stationmaster-like antenna using 1/2 hard line that would yield gain and more vertical beamwidth.

Nonetheless, PRCGUY's antenna is well suited for public safety work. He has done an admirable job putting the pieces and documentation together.

My home and monitoring location:

 

[krazybob]

The PRV-35 is a unity gain antenna designed for control tower operations. It has an even smaller beamwidth of 10 degrees. I'd rather have at a minimum the respectable Telewave ANT125Fs with 2.5dBd of gain and a vertical beamwidth of 38 degrees.

http://www.telewave.com/pdf/TWDS-7051.pdf

But you see, this points out what we're missing here. A control tower antenna does not need a large beamwidth. They are approach and departure. Enroute here in southern California is handled by SOCAL and they have ~15 low altitude sites and ~15 high altitude sites. These sites are used based on sectors. They are cutting up a large piece of piece of pie very effectively.

In regards to the PRV-35 it is a collinear design and I'll bet that if opened up we would find 2-4 electrical 1/4 wave elements; maybe a 1/2 wave element and a single 1/4 wave at the top. It's only 4.5 feet tall and very, very easy to make. The 4 exposed dipole might work fine considering a jet at FL40 is traveling at 500mph It would quickly pass the doughnut hole above the antenna.

I have built several collinear antennas and recently switched to using 1/2 hard line instead of coaxial cable. Regardless of the antenna chosen hopefully some ideas have been born here that will maximize your listening pleasure with minimal work. I personally love the exposed dipole array over a Station Master. If using 2" OD pipe one can make an omni-directional vertical.

 

[prcguy]

I had a typo, see Antenna Products DPV-35 which is unity gain with a 65deg beamwidth. More gain equals tighter beamwidth and a unity gain antenna could never have anything as tight as a 10deg beamwidth.

Link to DPV-35 specs: DPV-35: 118-136 MHz, Ground to Air, vertical polarized, omnidirectional

The stacked collinear made from 1/2 wave chunks of coax can be a bit messy to make (I've made several) and you need about twice as many 1/2 wave elements to equal the same gain as an exposed dipole array where the elements are optimally spaced, The exposed dipole can also achieve wider BW and probably enough to cover the entire aircraft comm band. You might get the needed BW for aircraft band it you made the collinear from 1 5/8 or larger Heliax.
prcguy

The PRV-35 is a unity gain antenna designed for control tower operations. It has an even smaller beamwidth of 10 degrees. I'd rather have at a minimum the respectable Telewave ANT125Fs with 2.5dBd of gain and a vertical beamwidth of 38 degrees.

http://www.telewave.com/pdf/TWDS-7051.pdf

But you see, this points out what we're missing here. A control tower antenna does not need a large beamwidth. They are approach and departure. Enroute here in southern California is handled by SOCAL and they have ~15 low altitude sites and ~15 high altitude sites. These sites are used based on sectors. They are cutting up a large piece of piece of pie very effectively.

In regards to the PRV-35 it is a collinear design and I'll bet that if opened up we would find 2-4 electrical 1/4 wave elements; maybe a 1/2 wave element and a single 1/4 wave at the top. It's only 4.5 feet tall and very, very easy to make. The 4 exposed dipole might work fine considering a jet at FL40 is traveling at 500mph It would quickly pass the doughnut hole above the antenna.

I have built several collinear antennas and recently switched to using 1/2 hard line instead of coaxial cable. Regardless of the antenna chosen hopefully some ideas have been born here that will maximize your listening pleasure with minimal work. I personally love the exposed dipole array over a Station Master. If using 2" OD pipe one can make an omni-directional vertical.

 

[krazybob]

That was a big brain fart on my aide for sure. I popped it into Google and it said 10 degrees. Clearly I am mistaken. But what is its cost? Tessco and Talley doesn't list them. My cost on the Telewave ANT125F2 is $605. List is $864. I think as we close this thread out we should stick to what is reasonably available to the average listener.

I think my overall point still holds true. While we try to hear everything from one location they have 30 or so remote locations for the Los Angeles region. I am roughly 100 miles by air from LAX and they only come in so-so using a discone. If I take an older model TV antenna (not digital) I can actually hear LAX better. Most bands don't resonate on the "Scanner Beam" sold by Grove.

The collinear is very easy to make using hard line and limiting it to a gain of 3dB. I've seen some rip the dialectric and center conductor out of coax and slide it into brass cylinders, but why? LOL. Hard line is just that -- hard. Others have cut coax into lengths and soldered away. Why? Use hard line.

But the easiest to make would be a 1/2 coaxial sleeve antenna. I've made several and it is a 1 hour project if you have the parts on hand. 6 Meter Repeater Antenna 1/2 Wave Coaxial Vertical Antenna Design Just scale it down for the air band. The assembly shown is my CHP antenna and it kicks butt even though they have almost all switched to repeaters.

You're after gain and that always narrows the signal aperture. For distant communications your antenna would rock. From up here it may really rock. But we have to remember that the average person in Los Angeles lives at an elevation that yields about 40 miles to the horizon. Not to mention that the FCC grants licenses generally on a 40 mile coverage and less at 800MHz.

I'd love to compare actual reception using the DPV-35 -- I got it right that time LOL. My examples are just good for local coverage. But we're talking about airborne communications. My examples tend to receive skywaves better than an antenna with 9dB gain side-mounted to a tower. Refer to the image above with the boat. That is my configuration on my repeater antennas and I know from 1st hand experience the effect of a 16 degree beamwidth. A HT down in Redlands is not as effective into my repeater as the same HT in Temecula (yes, our repeater has great receive!)

A Uniden BC785 scanner has a sensitivity of 1.5uV or worse as measured on my HP8924c. Even the Bendix KX-99 is just under 1uV. The two main differences are that the scanner does not use true AM demodulation (and the KX-99 does), and too much gain introduces birdies and images from the FM broadcast band. On a discone with 20 feet of RG6Quad I can hear it just trying to come in. I wonder if the poor sensitivity is linked to this issue?

Enough said. We each have a preference and those that want to build your antenna for the air band sure can have at it. But as I've said your antenna will truly rock where it is intended to play -- 138MHz and up for ground communications. At the end of the day it is up to the listener and what they can build.

 

[prcguy]

My DPV-35 is just sitting in the garage rafters since I have a dual band VHF/UHF version up. I think the average listener could afford a hundred dollars for mine.
prcguy

That was a big brain fart on my aide for sure. I popped it into Google and it said 10 degrees. Clearly I am mistaken. But what is its cost? Tessco and Talley doesn't list them. My cost on the Telewave ANT125F2 is $605. List is $864. I think as we close this thread out we should stick to what is reasonably available to the average listener.

I think my overall point still holds true. While we try to hear everything from one location they have 30 or so remote locations for the Los Angeles region. I am roughly 100 miles by air from LAX and they only come in so-so using a discone. If I take an older model TV antenna (not digital) I can actually hear LAX better. Most bands don't resonate on the "Scanner Beam" sold by Grove.

The collinear is very easy to make using hard line and limiting it to a gain of 3dB. I've seen some rip the dialectric and center conductor out of coax and slide it into brass cylinders, but why? LOL. Hard line is just that -- hard. Others have cut coax into lengths and soldered away. Why? Use hard line.

But the easiest to make would be a 1/2 coaxial sleeve antenna. I've made several and it is a 1 hour project if you have the parts on hand. 6 Meter Repeater Antenna 1/2 Wave Coaxial Vertical Antenna Design Just scale it down for the air band. The assembly shown is my CHP antenna and it kicks butt even though they have almost all switched to repeaters.

You're after gain and that always narrows the signal aperture. For distant communications your antenna would rock. From up here it may really rock. But we have to remember that the average person in Los Angeles lives at an elevation that yields about 40 miles to the horizon. Not to mention that the FCC grants licenses generally on a 40 mile coverage and less at 800MHz.

I'd love to compare actual reception using the DPV-35 -- I got it right that time LOL. My examples are just good for local coverage. But we're talking about airborne communications. My examples tend to receive skywaves better than an antenna with 9dB gain side-mounted to a tower. Refer to the image above with the boat. That is my configuration on my repeater antennas and I know from 1st hand experience the effect of a 16 degree beamwidth. A HT down in Redlands is not as effective into my repeater as the same HT in Temecula (yes, our repeater has great receive!)

A Uniden BC785 scanner has a sensitivity of 1.5uV or worse as measured on my HP8924c. Even the Bendix KX-99 is just under 1uV. The two main differences are that the scanner does not use true AM demodulation (and the KX-99 does), and too much gain introduces birdies and images from the FM broadcast band. On a discone with 20 feet of RG6Quad I can hear it just trying to come in. I wonder if the poor sensitivity is linked to this issue?

Enough said. We each have a preference and those that want to build your antenna for the air band sure can have at it. But as I've said your antenna will truly rock where it is intended to play -- 138MHz and up for ground communications. At the end of the day it is up to the listener and what they can build.