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Scanner / Receiver Antennas For discussion of any type of receiving antenna used by a scanner or receiver base, mobile or handheld.

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  #1 (permalink)  
Old 06-01-2009, 5:03 PM
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Default Full-Wave VHF vertical dipole working great!

I just put up a prototype full-wave wire dipole for VHF and it is working great - thought I'd share some thoughts before I commit this to tubing for the elements.

It is extremely easy to make, and has a direct connection to the 75-ohm feedline without any impedance transformer. I'm now trying a few different prototypes cut for different bands and they are all working as expected. Like the ocfd it is fed off-center, but it is not really an "ocfd" designed for multiband operation. Essentially it is a 3/4 wave over 1/4 wave antenna.

I started out in the aircraft band centered on 127 mhz:

1) Overall length for full-wave: 936/127 = 7.37 feet or roughly 88.5 inches total.

2) Cut the wire at the 1/4 wave point for attachment to coax: 234/127 = 1.84 feet, or roughly 22 inches. Center conductor of coax to long leg.

So after the cut I have a 66" length and a 22" length, which is a little over 7 feet tall. But I have the vertical room. Aircraft operations are great.

Since this is a full-wave, it has a cloverleaf pattern rather than the standard donut-pattern for a half-wave. This is good for me, since I can't mount the antenna very high, and need a little upward look angle to get above my neighboring surroundings. It is very similar to the directional pattern of a 1/4 wave groundplane, although it has a bit more gain than a 1/4 wave - about 4db.

(This is the reason the antenna is not really that popular commercially - but if you live down low like I do, why not provide some gain in the angles you can see?)

I topped it off with a simple rf-choke made out of 5 closely-spaced turns of the feedline coax wrapped around a plastic water drink-bottle near the feedpoint. I ran out of clamp-on ferrites, and was too lazy to build a pawsey-stub. Seems to be working well on my 2-meter amateur version of the antenna which is a little bit shorter.

(With the full-wave version on the amateur 2-meter band, I ran an mfj analyzer on it and got 3:1 swr on the band edges and about 1.5:1 on the center)

Anyway, I'm really happy and there don't seem to be any surprises compared to my 1/4-wave groundplanes - other than stronger signals! So the directional pattern seems to be holding like the books and modeling say.

Now I just need to put these in the attic or maybe on the side of the house eves - but amazingly it is working very well indoors near ground level.

(Need to take a break since with a little bending, you can turn this into a vertical "half-square" with some directivity....another day perhaps...)

Last edited by hertzian; 06-01-2009 at 5:10 PM.. Reason: typos
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Old 06-02-2009, 7:17 PM
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Day two: still great results from essentially two half-waves butted against each other in a 3/4 over 1/4 feed to get that cloverleaf pattern.

This is one antenna I wouldn't be ashamed to take to the hill for transmit use considering that you can achieve a very good match to coax.

Since my listening consists mostly of aircraft and hilltop repeater systems, the little bit of "uptilt" is really working well.

The larger of the lobes are around the long end, so I mount it down in the valley with the long end upwards.

If I were to take this to the hill, where most commercial antennas have some sort of phasing "downtilt", I'd mount this antenna on the hill with the long-end facing down.
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Old 06-03-2009, 5:31 PM
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Do you have any pictures you can post? I used old tent poles to experiment around with dipoles & ocfd
with results that seem equal or slightly better than all the antennas I have ever bought , but then Im pretty close to most things I want hear radio wise.
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Old 06-04-2009, 4:20 AM
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Not much for pics as it is just a 22" inch wire and a 66 inch wire connected directly to 75 ohm coax. My "shack" is a table in the living-room. Maybe when I commit this to tubing and something to be proud of.

Let me try to attach some plots from EZnec. (highly recommended!) I'll post the elevation angles over somewhat realistic ground, in theoretical free space, and also the swr plot for 75 ohms so you can see the cloverleaf pattern that I desire to aim towards the hilltops and aircraft when this antenna is mounted low. Believe it or not, it is performing well with just the feedpoint up at about 4 feet above ground! I really should put it at least a bit higher.

Also note that modeling is one thing, and real-world is another - so I usually get into some pretty heavy listening sessions to make sure I'm getting the expected results. So far so good!
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Last edited by hertzian; 06-04-2009 at 4:25 AM..
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Old 06-06-2009, 2:59 PM
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Default Performing better than expected - why?

For general aviation use in 118-136 this antenna is performing much better than expected - which bothers me based on the modeling. Like that's a problem!

My "problem" is that I am used to the overhead null with a half-wave dipole, but I am not getting that deep of a null with this one, which based on the modeling, should be even wider with this full-wave version. I'm used to hearing overhead planes fly through the null going from strong/weak/strong. Obviously it is hard to pin down an overhead moving object, but after a lot of listening and taking another good look at the antenna I think I have finally gotten in the ballpark!

What I haven't included in the modeling is a THIRD wire. That is, the coax feedline coming away from the antenna feedpoint is not choked and serving as a common-mode single-wire groundplane. So essentially the "real" antenna is much different now that I've included it as part of the antenna (the outer "skin-depth" of the coax braid is what the rf is seeing). I actually choked it at a quarter-wave down the line and now things are starting to line up.

Here are some plots showing the real antenna, and the elevation plot for 127 mhz, which is very similar across the band:
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Last edited by hertzian; 06-06-2009 at 3:50 PM..
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Old 06-06-2009, 3:11 PM
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Default Multi-banding for mil-air 225-400 mhz!

Here is another twist! I now have mil-air coverage as well by purposely using a 4:1 tv-type impedance transformer (your typical 300:75 ohm "balun")

While a direct connection to 75 ohm coax provides a great match within the 118-136 mhz band, it left a lot to be desired from 225-400. I compromised and the 4:1 balun provides a wider response, although quite a bit reactive, however for rx-only purposes, it is doing just fine!

This also eases construction for those who want to DIY, but just don't want to hassle with cutting coax, or be constructed by children or others who really shouldn't be playing around with knives. I've been careful and cut my hands many times over the years doing antenna work, so this is a consideration for safety - using the 4:1 balun and just connecting things together works just fine.

Here are the plots for an assumed 300-ohm impedance at 127 mhz and 250mhz, and also the elevaton plots for mil-air 225-400.

I consider the mil-air coverage as just "pot-luck" since the lobes are so unpredictable. However, I am now hearing mil-air with the 4:1 impedance transformer much better than a direct-connect to coax!

Yes, for real mil-air use, a dedicated antenna is the way to go, but if you can live with it, go for it! You can see the effect of the "3rd-wire" sloping the main lobe to the left a little bit - or attenuating lobe on the right. I can't tell - my ears aren't *that* good.
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Last edited by hertzian; 06-06-2009 at 3:52 PM..
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Old 06-06-2009, 3:45 PM
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Default Additional thoughts

Part of what was throwing me off initially is that I make a habit of choking the feedline right at the feedpoint, and when I constructed this one, I put it together hastily and forgot to choke it. That was actually a good thing!

Now that I am choking the feedline at a 1/4 wave, 22-23 inches away horizontally, it is all starting to come together. One could run without a choke and expect something a bit similar, but you never know.

If you want to experiment even further, especially if you want to try to force the lobes into a different pattern, one could actually fold the short 1/4 wave vertical element horizontal to nearly emulate a two-wire groundplane! Or even add more 1/4 radials and see what happens.

If you want to get started with EZnec, a simple vertical dipole even with the demo version is quite handy. It is likely you'll want the non-demo version if you desire more accuracy, or try to model more complicated antennas with perhaps hundreds of elements (electrically) like the discone.

EZNEC Antenna Software by W7EL

In the real-world, I doubt my plots are exactly what the antenna is seeing, but I think I've gotten close enough to satisfy my curiosity.
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Old 06-06-2009, 4:14 PM
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You say your monitoring the airband , I wonder if you've ever tried ACARS - Free ACARS decoder for Linux & Windows - acarsd I dabble with it occasionally , The reception is typically quite a bit further than voice , because it decodes automatic messages generally from enroute altitude , It can also show you the aircrafts position on a map you can make for your area , & displays a picture of the type of aircraft you have made contact with ,although I cant seem to get the mapping to work correctly , ( im doing something wrong) it is still pretty interesting & I wonder just how far you could pick something up with , with your designs.
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Old 06-06-2009, 4:26 PM
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Thanks - I'll have to check that out as I also run Debian GNU/Linux on my Apple G5 ppc. I guess I love challenges.

Once I took the common-mode feedline braid current into account, this is very different from a true vertical dipole. If one bent the short element hanging down back up to either horizontal or 30 degrees or so, it quickly becomes apparent that this is not a dipole at all, but more properly a 3/4 wave groundplane! At least that is how it seems to be acting.

It has been a lot of fun that's for sure!
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