Airband Antenna Musings

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N9JCQ

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I spent part of the weekend updating the frequencies in my primary Civil/Milair scanners and it got me thinking about the antennas I use. As background, I have three scanners I use for Civil/Milair in my shack and portable use. This first is the venerable PRO-43. I am now on my third Pro-43 (Thanks Scott!) and it is by far the best portable aviation scanner out there bar none (In my opinion of course).

The second scanner I have is the Yupiteru MVT-7100 which follows close behind the '43. It has the memory I require for the various locations I travel to. Finally, for ease and flexibility, I have a Uniden BC396T which I find to be a good overall air band scanner and great for those times when you want to hear a lot of varied things. I use this scanner in the car a lot paired to a Niljon Super M mobile antenna. When I travel in the car, because of it's capacity, the uniden is the scanner I use.

Now to the antennas. I am an antenna junky. I know, that the key to reception is the antenna, more so than the scanner attached to it. As an example, the Pro-43 is in my opinion the top dog. However, if you use the stock antenna, it is basically deaf on Civil/Milair unless you are in a couple of miles of the airport. You will certainly copy aircraft in flight, but not in any great distance.

Now, if you take the 43 (or 7100, or 396T) and connect a Diamond RH-77 (or SRH-77 for the 396T), you now have something that I consider adequate for monitoring aircraft. From my home, I can easily copy O'hare's towers and ground control freqs from 20 some miles away over relatively flat ground. Air coverage is greatly increased for Civil and Miliair over stock antennas. I can usually copy ACM maneuvers in Central Indiana on 350.350 AM and this is quite a distance from me.

Although it's been written here that the Maldol AL500H air (civil and Milair) is comparable to the Diamond RH-77, in my case, it is not. I find the Maldol to be far superior to the Diamond, especially for O'hare and Chicago Center and Approach/Departure traffic. For air/air, it also is several s units stronger than the Diamond using the Yupi and 396T.

However, the Maldol is pricey at $44.00 or so and may be priced out of the average scanner listeners price range. Also, the Maldol is long and gangly. Certainly you cannot use the scanners on a belt with this antenna attached. You have to leave the scanner on the table. However, in my use, it is a significant improvement over the Diamond RH-77. The Maldol is not as broadbanded so it is not effective on 800MHZ as the Diamond is.
Ultimately though, the duck and collapsible antennas I have tuned for the Civil air work best for 118-137 MHZ. They are tuned for the civil air band only and work best for that application only. Your mileage may vary.
 

eagleswings01

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Have you tested the Diamond RH-77 against the RS 20-006? I currently have the RS on my PSR-310, and although it is much better than the stock ducky, I think the Diamond will be a better upgrade.
 

nanZor

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I've tested them and agree for the most part with N9JCQ's findings. Problem is, even if you want a Maldol AL-500H, they are discontinued from what I can tell.

Aside from field usage testing, I've also placed them on a Comet CAA-500 analyzer to see just where they are most resonant. While resonance isn't a guarantee of efficiency (you can load a paperclip for low swr and have an inefficient little stub), I can see where the swr and impedances go way above 100 ohms, and far below 35 ohms for a reasonable range.

The Maldol was right-on resonant at 128 and 300 mhz respectively, The RH77 was nowhere near resonant in the VHF airband, and was happier at the higher end of 2m amateur, really about 152 mhz or so. I'd take the Maldol any day provided you can deal with the whippy length of it.

Have you tried this with your 20-006: Instead of extending it all the way, try extending it fully, and then push the element just above the coil back down about 1/4 to half-way downwards until you feel some resistance as it hits the connection inside that bypasses / shorts the coil. Don't push that element ALL the way down inside - just a 1/4 to 1/2 length will do. I've noticed improved performance on vhf airband this way, and the Comet analyzer shows a big difference. Fully extended, the Comet analyzer shows resonance at about 100 ohms impedance, whereas if you short the coil out by pushing the element above the coil just down inside it a little bit to short it out, the swr goes straight to 1:1, and the impedance hovers near 50 ohms. Try it on a stable signal like a local ATIS and see what you think.

Unfortunately the Comet analyzer does not indicate the difference between a resistive and a highly reactive impedance. But from my own comparisons, shorting out the coil seems to indicate that the "shorted" version of the 20-006 may be a nicer 50-ohm resistive load, whereas the fully extended 100-ohm load may be quite a bit reactive. If I hadn't blown up my MFJ analyzer, I could tell a lot easier.

I'm not a 1:1 swr junkie - especially on rx-only ducks - but I like to know where the resonant points of the antennas I use are. :)
 
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majoco

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Make a vertical dipole for the civil aviation band which you feed to one scanner and another for the mil band which you feed to another. The 2 to 1 frequency difference is too much for a simple antenna.
 

nanZor

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One consideration to look at is bandwidth vs desense / overload issues on inexpensive scanners. In this case, a wide bandwidth is unwanted.

The Maldol AL-500H is tuned to 128mhz and 300 mhz as confirmed on an analyzer. Yet like you mentioned, there is no way it can have that huge kind of bandwidth for milair on a very thin wire. But, this narrow bandwidth can be very beneficial to those with overload from FM broadcast, NOAA, other UHF sources etc with a $100 scanner and restore sanity to milair monitoring, even though it isn't the greatest at the milair band edges.

Quick alternatives, although not the absolute best are to just use a commonly available 2M, 220 mhz amateur duck, preferably a long-ish whip and see if it fits your needs. Another for milair only, is to use a 220 mhz amateur antenna, mostly working best at the low-end, 220-250 mhz or so.

For those suffering overload / desense, and still want to use a duck or small whip, do a comparison between the canonical Diamond RH77CA, and the Maldol AL-500H on FM broadcast, NOAA, Airphone UHF carriers etc. The out of band rejection of the Maldol smokes the RH77. :)
 
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Halfpint

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For those suffering overload / desense, and still want to use a duck or small whip, do a comparison between the canonical Diamond RH77CA, and the Maldol AL-500H on FM broadcast, NOAA, Airphone UHF carriers etc. The out of band rejection of the Maldol smokes the RH77. :)
Amen! When I first heard about the Maldol AL-500H I was somewhat skeptical about all the claims. After using one on several assorted scanners over the years I've become `sold' enough that I have a couple set aside as `spares' and a couple I use pretty much full time for my air band scanning. I don't have all the problems I used to have with the 3 different FM broadcasters that almost completely surround me and they even are nice whenever I make it ot any of the local air shows that are quite as surrounded as I am at home. I *will* admit that sometimes their length can be a bit of a problem but, the end results are definite worth while. The RH77s are now in my tool cabinet drawer along with all the other antennas I've collected over the years and seldom come out. Usually as a `last resort' when I'm having problems listening to things other that aviation.
 

nanZor

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I guess this is heading into antenna-forum territory, but at least it has some airband monitoring stuff.. :)

Note that while testing, if the broadcaster is in your backyard like it is mine, I have to enable the attenuator on my 396xt to actually discern the difference on FM broadcast between the RH77 and the Maldol. If tested while unattenuated, you may not notice the difference. So in my case, the Maldol didn't make the offending fire-breathers go away completely, they were just attenuated enough by the Maldol's narrowbanding so that desense doesn't take place, and I can listen to airband(s) unattenuated under normal circumstances. Sounds just like your situation.

The RH77s are now in my tool cabinet drawer along with all the other antennas I've collected over the years and seldom come out. Usually as a `last resort' when I'm having problems listening to things other that aviation.
My last-resort antenna for scanners with decent front-ends that can handle a bit of bandwidth, is the Diamond SRH320A duck. I prefer it over the RH77. Covering 2m, 220, and 440 mhz amateur naturally it is resonant there. But I have also found it does a great job - even better than the RH77 for VHF airband, 2m, and 150-170 mhz. It shows it too on the Comet analyzer, although swr isn't everything, especially when I don't know if I'm dealing with a reactive or resistive load. But I would have to say that Diamond did something with the 320 to make it perform better then the 77's claims. Even for milair, the 320A seems to have even a bit better bandwidth up to about 300 mhz as compared to say a standard 220mhz amateur duck.

I'm not a duck fanatic, but the SRH320 is my favorite all-rounder since it covers a large portion of VHF really well, and even the low end of milair within reason - and 420+ uhf is fine, but I didn't really do much testing up there. BUT, it does NOT have the FM broadcast band rejection that the Maldol does - close maybe, yet it still has better FM broadcast band rejection than the RH77! Too bad they don't make the AL500H any more.

In the end though, these are just ducks - my crappy homemade dipoles beat them all! :)
 
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nanZor

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More FM Broadcast rejection testing

Armed with my Comet analyzer, and using an Icom R20 this time switching between my local AWOS as a stable reference and my fire-breather on 88.5mhz FM , I did some more testing and found that another commonly available antenna has good FM broadcast band rejection: The RS center-loaded telescopic whip - BUT ONLY when fully extended.

Even though when fully extended it is resonant at around 45 mhz, it is also passable for VHF airband on the 3rd harmonic. Certainly much more so than the typical oem duck. For airband, normally I disable the loading coil by pushing the element just above the coil down just a little bit until you feel it make some contact with the internal fingers, thus disabling the coil and becoming just a standard telescopic which provides a bit of a better signal for vhf airband. The Comet shows the improvement too.

HOWEVER, there is a catch - when you disable the loading coil, you lose the FM Broadcast rejection. So, if you are having problems with FM desense / overload and are using the RS telescopic, you may have better results by actually leaving it fully extended with the coil inline, even though this isn't the best match per se.

This leads me to believe that if a duck has a center-loaded coil, the reactance of that coil out of band is helping it do a better job of FM Broadcast rejection. The Maldol AL-500H has one. So does the Diamond 320A. And so does the RS center-loaded whip, but only when fully extended.

Another alternative to these long ducks, is to use a commercial duck designed for pilots like those found on the Icom and Yaesu/Vertex. (Icom FA-B02AR bnc mount, and Yaesu/Vertex ATV-10 airband model, sma mount). I have found that they are a great way to keep things short, and perform better than even an off-tuned amateur antenna. The Icom R20's signal strength meter shows a significant difference, something I'd never see on a Uniden 396XT.

BUT, there is another catch for the airband scanner user - the specifically tuned ducks for commercial airband use, also don't have good FM Broadcast band rejection! With a cheap front-end, if you are suffering from desense / overload, these nice commercial airband-ducks won't help you much.

So there you have it - for the antennas I've owned, the Maldol is tops in out-of band rejection both above and below it's center frequencies. The Diamond 320A has good FM broadcast band rejection, but it more or less wideband above. The RS telescopic has good FM Broadcast rejection, but only when fully extended, and may not be ideal for weak-signal vhf airband work that way.
 
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sholt

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Hi everyone,

This thread has made for some really good research as someone fairly new to antenna research and interested in listening to VHF air band.

I posted a question or two on the Maldol AL500H product review page and, if you have a moment to have a look and help me out I'd really appreciate it! http://forums.radioreference.com/sc...9612-maldol-al-500h-vhf-uhf-airband-whip.html

In short, I am looking at improving my air band reception and was not sure, given my situation, location, etc if I should be making an effort to find an out-of-production Maldol or if the RH77CA (or even the 320?) would do the job for me.

Thanks for your help :) , apologies if a cross post is frowned upon :-(
 
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