Best Comm Antenna

[Rob58]

What is the real performance difference, if any, between the different models of comm antenna? Do the low profile antenna designs - those with the bent element - sacrifice any performance? Can any available models be tuned to optimize SWR? Thanks for any feedback!

 

[bluerooster]

I have a fiberglass Commant broadband antenna on comm 1, and the origional wire whip on comm 2.

The Commant exibits a 1:1 SWR accross the frequency range. The whip shows 1.2:1 on 122.8, and changes at lower frequencies, but not much. mabe about 1.3:1 on 121.0. In order to tune it on lower frequency it would have to be lengthened. (no way to do that) SWR of 1:1 is obtained at 123.(something) But I forget the exact frequency. I figure anything below about 2:1 is acceptable.

The transistor finals will start to reduce power output at about 1.8:1, to keep from toasting the final amplifier due to too much reflected power.

 

[AA5Bman]

This is something I’ve never thought about. I have a pretty crappy Comm that doesn’t seem to broadcast with much range, could it just be the antenna? Are you allowed to just swap antennas? I always figured the Comm and the antenna were kind of a package.

 

[IK04]

Any antenna that is tuned for the AM Air band will work, as long as it has the same connector (BNC) it will work.

 

[denverpilot]

This is something I’ve never thought about. I have a pretty crappy Comm that doesn’t seem to broadcast with much range, could it just be the antenna? Are you allowed to just swap antennas? I always figured the Comm and the antenna were kind of a package.

It could be the antenna or even a bad coax or connector. This can all be tested and swept by a competent avionics shop to find the problem. (I’m not one and even I have all the tools needed to sweep a VHF antenna in my radio shack/electronics workbench room at home.)

The antennas aren’t radio specific in any way, other than certain GPS antennas.

As for the guy with the short whip... jokes waiting to be made there... the whips often have a screw that holds the whip in place and the whip itself can sometimes be replaced without changing out the entire antenna. Can buy a longer one and cut to tune, with the right test gear.

 

[bluerooster]

Antennae are easy. You can build your own from scrap materials, but For a certified aircraft it gets more complicated than that.

Yes, without a good antenna, the very best radio ain't worth squat, and with a good antenna, the worst radio can perform quite well.


I talk to my dad, every evening from GA to TX with a scrap of wire between two trees for an antenna.

 

[denverpilot]

I talk to my dad, every evening from GA to TX with a scrap of wire between two trees for an antenna.

Wish I could talk to my dad on the radio again. Cherish it. I only have some recordings of him working other folks now.

Dad was KC0VFO. Got that assigned even. Wasn’t a vanity.

I let the callsign go and it’s now held by a young man who was a friend of his.

73 DE WY0X

 

[weirdjim]

Excuse, please, i just wrote all this stuff up for my December column. I'll post the salient parts when I get home from OSH.

Jim

 

[weirdjim]

ok, here we go.

The input impedance of a quarter-wave (22") whip antenna is 32 ohms at the base of the antenna. This is a 1.5:1 VSWR right off the crack of the bat and there is no known simple way other than a transformer to make it any better. Nobody I know imbeds a transformer in the base of their antenna, so anybody that tells you that they have a 1:1 VSWR on their antenna is either spoofing you or smoking funny smelling weeds.

That is at the EXACT band center of the antenna. It only gets worse above and below band center. How much worse? It is a function of the FATNESS of the antenna. The old wire whips that had a 3/16" stainless rod were below 2:1 or so over a relatively limited band of frequencies ... say, 118 to about 124 with a band center of 121 or so.

But with fatness comes bandwidth. Some manufacturers of those fiberglass rod antennas stuff a thin copper sheet or 1/2" wide tape into the rod and pop that 2:1 bandwidth up to 118-130 (band center 124 MHz.) and if they get REAL generous and use 3/4' tape they can get the bandwidth below 2:1 118-137 MHz. (band center 127 MHz.).

But what exactly does VSWR do to you. Let's take the example of a 1 watt transmitter at various VSWRs (just multiply by the actual power output of your radio).

VSWR : 1.........power reflected.........watts reflected.....watts transmitted

1.5........................4 %....................40 MILLIwatts.............0.96
2.0........................11%..................110 MILLIwatts........ ....0.89
2.5........................18%..................180 MILLIwatts.............0.82
3.0........................25%..................250 MILLIwatts.............0.75

This whole argument of a 1:1 VSWR goal reminds me of the carpenter that uses a micrometer to measure framing lumber. His neighbors just think he is a doddering old fool and his fellow carpenters think he is one taco short of a combination plate.

Most VSWR protection circuitry in anything below 10 watts output is fairly standard at a 3:1 kick-in. When I designed the RST-571/572 navcoms I did enough heat sinking that you could run them short/open circuit and be WELL within thermal and second-breakdown limits.

Considering the fact that a 1 watt transmitter and a 1 microvolt receiver have a theoretical range of just shy of 200 miles renders the entire argument moot.

Thanks,

Jim

 

[weirdjim]

Antennae are easy. You can build your own from scrap materials, but For a certified aircraft it gets more complicated than that

There is no such thing as a certified aircraft. Certificated, to be precise. Some experimental, some normal, some utility, and some aerobatic.

Jim

 

[denverpilot]

ok, here we go.

The input impedance of a quarter-wave (22") whip antenna is 32 ohms at the base of the antenna. This is a 1.5:1 VSWR right off the crack of the bat and there is no known simple way other than a transformer to make it any better.

This is fascinating because I have seen 1/4 wave antennas showing about 1.2:1 to the radio with really good test gear. I assume the transformer, or just flat out heat loss on the return, must be happening in the coax. But have seen this with hardline with nearly no loss at VHF also. UHF always has some loss in most of the hardline I use unless we start playing with the 2” stuff or larger.

If it were measured at the antenna end, I assume it would match your numbers. Interesting.

 

[weirdjim]

This is fascinating because I have seen 1/4 wave antennas showing about 1.2:1 to the radio with really good test gear. I assume the transformer, or just flat out heat loss on the return, must be happening in the coax. But have seen this with hardline with nearly no loss at VHF also. UHF always has some loss in most of the hardline I use unless we start playing with the 2” stuff or larger.

If it were measured at the antenna end, I assume it would match your numbers. Interesting.

Sure, if I put a thousand feet of RG-58 on the 1.5:1 antenna, the simple loss of the coax would bring the VSWR down to less than 1.5, closer to 1.1 or 1.2. Whoever is measuring 1:1 on their antenna is probably using a CB reflectometer good at 27 MHz (good buddy) and with the concomitant loss in frequency disparity is showing 1:1 as a specious result.

Jim

 

[Timbeck2]

Um...how about for those of us who don't know a 1/4 wave antenna from a Spiderman lunchbox...who makes a good antenna/model number for GA aircraft use?

 

[weirdjim]

Um...how about for those of us who don't know a 1/4 wave antenna from a Spiderman lunchbox...who makes a good antenna/model number for GA aircraft use?

Not the focus of this newsgroup question. You want Spiderman vesus lunchboxes there is a forum for that. Now, if you want to find out who makes a good antenna and model start that question in another forum subject.

Jim

 

[Timbeck2]

Not the focus of this newsgroup question...

Jim

Neither was the guy who asked about "certified aircraft" Jim but you sure went out of your way to answer.

 

[denverpilot]

Sure, if I put a thousand feet of RG-58 on the 1.5:1 antenna, the simple loss of the coax would bring the VSWR down to less than 1.5, closer to 1.1 or 1.2. Whoever is measuring 1:1 on their antenna is probably using a CB reflectometer good at 27 MHz (good buddy) and with the concomitant loss in frequency disparity is showing 1:1 as a specious result.

Jim

Nope. 100’ of 7/8s hardline, Anderson with proper connectors, and measured with an HP service monitor and confirmed with a Bird.

Can only report what I’ve seen. These are quarter waves with groundplane radicals bent down at the usual angles for such things. Not aircraft skin, of course. Don’t need 100’ of hardline for that.

Never EVER seen any better than roughly your 1.5:1 theoretical number on any vehicle though. Cars, trucks, stuff with many feet of flat metal roof (trailers/RVs).

But go up a tower 100’ and it’ll look much better. Not quite sure why. Ground effect really isn’t in serious play.

 

[weirdjim]

OH, you put the antenna as a groundplane with the "usual angles" for such things. You are approximating the difference between a true ground plane at zero degrees relative to the radiating element and the groundplane build down at about a 45 ° angle. Guess what you just did? You did the split between a zero angle groundplane antenna and a dipole (if you were to run the groundplane elements) down to zero degrees. At 45°you split the difference between the ground plane (32 ohms) and the dipole (72 ohms) and match 50 ohms nearly perfectly. Nice job.

Jim

 

[denverpilot]

That’s what I thought that the bent ground plane changes the resistance value at the antenna...

Thanks.

 

[weirdjim]

That’s what I thought that the bent ground plane changes the resistance value at the antenna...

Thanks.

Yeppers, you are half ground plane at 32 ohms and half dipole at 72 ohms, add the two, divide by two you get 52 ohms which is a 1.04;1 VSWR. Bend them up a little and down a little to match perfectly.

Which is why com antennas near the tailfeathers on the top of the fuselage match so well. The fuselage slopes off at a 90° angle and if you hit it just right you get a perfect match.

Jim

 

[weirdjim]

That’s what I thought that the bent ground plane changes the resistance value at the antenna...

Thanks.

VEEEERY slight correction. Resistance is measured in ohms, but it has a characteristic that is the same at DC as at high frequencies. Impedance is measured in ohms, but it can measure zero or infinity at DC, but a very specific impedance in ohms at a specific frequency. Impedance <> Resistance.
Thanks,

Jim

 

[bluerooster]

Neither was the guy who asked about "certified aircraft" Jim but you sure went out of your way to answer.

I figured he was just splitting hairs.

 

[bluerooster]

Resistance is generally DC, and Impedance is generally AC. Along with impedance comes reactance, which can be either capacitive, or inductive.
Both, resistance, and impedance, quantify the opposition to electromotive force in a circuit in ohms.

 

[weirdjim]

I figured he was just splitting hairs.

That's what engineers do for a living.

 

[weirdjim]

Resistance is generally DC, and Impedance is generally AC. Along with impedance comes reactance, which can be either capacitive, or inductive.
Both, resistance, and impedance, quantify the opposition to electromotive force in a circuit in ohms.

Sort of. A carbon resistor (as far as the stray capacitance and inductance will let it) is a resistance well into the AC and RF world. An impedance has both resistive and reactive components ( r+/-jx, remember???).

Let's not let this devolve into a catfight into the subtleties of the resistive/impedance thing. That is MSEE 201 and I've been teaching it for over 40 years. THis is a beginners antenna forum. Let's keep it that way.

Jim

 

[denverpilot]

VEEEERY slight correction. Resistance is measured in ohms, but it has a characteristic that is the same at DC as at high frequencies. Impedance is measured in ohms, but it can measure zero or infinity at DC, but a very specific impedance in ohms at a specific frequency. Impedance <> Resistance.
Thanks,

Jim

I understood but good to post it for completeness here. I just typed resistance because I was tired.

Cool trick mounting the antenna near the tail... hadn’t thought of that one.

Ours are up top on the wing where it’s flat so probably not the best match. Never measured it though. They’re the somewhat “fat” fiberglass sort which sounds like they do a larger surface area just wrapped around the fiberglass, eh? Interesting.

Actually now that I think about it they’re not fiberglass. They’re metal because there’s a spot on one that the paint was rubbed off of, and it looks like (long ago) someone prepped and painted it. Very small spot.

Never had a lick of trouble out of either one of them, but it’s interesting that you can match them better near other geometries of the airframe other than “put it on top on the flat part”.

Now you have me wondering if they change their match as my large flaps come down. Hahaha. Probably too far away physically at VHF from the mount point to make any real difference, but it made me laugh to think about it. Could even make them a teeny tiny bit directional? Not that it matters, it would be so small.

Rather be flying than contemplating antenna matches for antennas with thousands of feet of height over average terrain most of the time... if they can’t hear us from up high, something is seriously wrong.

 

[bluerooster]

Sort of. A carbon resistor (as far as the stray capacitance and inductance will let it) is a resistance well into the AC and RF world. An impedance has both resistive and reactive components ( r+/-jx, remember???).

Let's not let this devolve into a catfight into the subtleties of the resistive/impedance thing. That is MSEE 201 and I've been teaching it for over 40 years. THis is a beginners antenna forum. Let's keep it that way.

Jim

Got it.

 

[Stephen Poole]

Yeppers, you are half ground plane at 32 ohms and half dipole at 72 ohms, add the two, divide by two you get 52 ohms which is a 1.04;1 VSWR. Bend them up a little and down a little to match perfectly.

It's also possible to make the antenna a little longer than 1/4 wave to up the resistance. This makes it a bit inductive, so a small capacitance is added to make it resonant. Not a lot of bandwidth, though, as you point out above.

Which is why com antennas near the tailfeathers on the top of the fuselage match so well. The fuselage slopes off at a 90° angle and if you hit it just right you get a perfect match.

Jim

I wonder if the manufacturers consider that when they're choosing antenna placement? I wouldn't want to bet on it.

-- Stephen (who has a 5-tower, 50KW AM directional array with 109 degree radiators ... with mutual impedances, it's a BUNCH of fun to tune ...)

 

[weirdjim]

-- Stephen (who has a 5-tower, 50KW AM directional array with 109 degree radiators ... with mutual impedances, it's a BUNCH of fun to tune ...)

I wouldn't want to tune that bitchkitty on a bet. WAY too many variables and screwing around with enough power to make me sizzle isn't my cut of pea.

Funny, though, we have a little 5 kW (daytime) AM popcorn popper in town and I accidentally turned on the AM receiver late one night/early one morning and heard a carrier no modulation. Just for giggles I drove over to the antenna site and saw a light on inside. I introduced myself and asked what was going on. They guy said he was a professional broadcast engineer and was checking the phasing of the two-tower array to be sure we were protecting the Minneapolis station (from California???) from night QRM. He was exciting the thing with a vacuum tube Johnson Viking 1950s era ham rig modified to tune the broadcast band with about 50 watts into the tower(s). Test equipment is where you find it.

Jim

 

[weirdjim]

It's also possible to make the antenna a little longer than 1/4 wave to up the resistance. This makes it a bit inductive, so a small capacitance is added to make it resonant. Not a lot of bandwidth, though, as you point out above.

That and the simple addition of xL and xC raise the Q still higher, which lowers the bandwidth, which raises the VSWR, which ...

Jim

 

[Rob58]

I am always amazed at where my innocent and simple questions lead this crowd - interesting reading for sure! By the way, thanks for the helpful responses to my OP.

 

[Stephen Poole]

By the way, I like your Website. Looks like you have some nice stuff!

I wouldn't want to tune that bitchkitty on a bet. WAY too many variables and screwing around with enough power to make me sizzle isn't my cut of pea.

The thing about a high powered array is that even the "idle" towers pick up enough juice to zap you. I've got a nice scar on my arm from when I was brazing some copper at one of the towers.

Nowadays we can use computer modeling, but not that long ago, it literally took weeks to get a multi-tower array tuned. Each time you tweak one tower, everything else changes. (With 109 degree radiators, just opening the doors on ATUs -- the metal boxes at the tower bases -- will cause the phases to shift). Make some adjustments, then run in all different directions with field strength meters to see what the pattern looks like. FUN.

That and the simple addition of xL and xC raise the Q still higher, which lowers the bandwidth, which raises the VSWR, which ...
Jim

Sure! And to be fair, I only deal with one frequency and a (relatively) narrow bandwidth, AM, FM or TV. Your stuff has to handle a much wider range of frequencies.

Hey! I know, an aviation discone antenna!!!

 

[Stephen Poole]

I am always amazed at where my innocent and simple questions lead this crowd - interesting reading for sure! By the way, thanks for the helpful responses to my OP.

Actually (*cough*), we kind of strayed, there. Were you specifically asking for a *brand* of antenna, or a type of antenna (vertical, L-shaped, etc.)?

 

[Rob58]

Actually (*cough*), we kind of strayed, there. Were you specifically asking for a *brand* of antenna, or a type of antenna (vertical, L-shaped, etc.)?

Oh yeah, what was my question... almost forgot. Very simple, I am replacing two comm antenna, one on the top and one on the bottom of the fuselage (not much ground clearance thus the L-shape). Just trying to understand if there is really any difference in what is available on the market. Also if there is any trick to tuning for the lowest VSWR. Remember I have a tube and fabric fuselage. Trying to make an informed decision. Of course feel free to go off in any random direction!

 

[Stephen Poole]

Oh yeah, what was my question... almost forgot. Very simple, I am replacing two comm antenna, one on the top and one on the bottom of the fuselage (not much ground clearance thus the L-shape). Just trying to understand if there is really any difference in what is available on the market. Also if there is any trick to tuning for the lowest VSWR. Remember I have a tube and fabric fuselage. Trying to make an informed decision. Of course feel free to go off in any random direction!

I'll let @weirdjim and folks who have actual PIREP experience speak to that. As for lowest VSWR, the owner's manual for the antenna should have the information on that. Jim is right about making sure you use a meter or analyzer that's good at aviation frequencies.

 

[weirdjim]

Oh yeah, what was my question... almost forgot. Very simple, I am replacing two comm antenna, one on the top and one on the bottom of the fuselage (not much ground clearance thus the L-shape). Just trying to understand if there is really any difference in what is available on the market. Also if there is any trick to tuning for the lowest VSWR. Remember I have a tube and fabric fuselage. Trying to make an informed decision. Of course feel free to go off in any random direction!

I guess the first question I would ask is why you are replacing the antennas?

Jim

 

[Rob58]

My existing antennas are old style, bare metal, without the fiberglass protective fairing. As such they do not anchor very well because they use only a single attachment point.

 

[weirdjim]

My existing antennas are old style, bare metal, without the fiberglass protective fairing. As such they do not anchor very well because they use only a single attachment point.

Ah, the old stainless steel whips, all the rage when our radios were 90 channel 118-124 MHz beasts. Now with the band going from 118-137 MHz. they lack a little in operation at the band edges.

However, they work really well when you are at a small town low-key flyin for roasting weenies over the campfire. Not so much for making smores, as the marshmallow sticks to the stainless.

You may be condemned to keep the belly mount antenna as I don't readily see a broadband option, although the RAMI AV-17 comes reasonably close. I've got AV-529s on my personal 182 and they have performed flawlessly for 30 years.

Did that help?

Jim

 

[IK04]

That and the simple addition of xL and xC raise the Q still higher, which lowers the bandwidth, which raises the VSWR, which ...

Jim

I love that stuff. Impedance matching and Q was my favorite part of earning my Amateur Extra.

I have used those calculations successfully in building my E/H loop antennas...

 

[weirdjim]

I'm going to try and take this conversation to another thread so we don't run this thread so far OT.

Jim

 

[Ghery]

I love that stuff. Impedance matching and Q was my favorite part of earning my Amateur Extra.

I have used those calculations successfully in building my E/H loop antennas...

Back in 1992 when I first passed the General, Advanced and Extra writtens, the Extra written was very easy. If you had been playing in the hobby and paying attention you could pass it without studying. Now, the Advanced written back then had questions on stuff I hadn't seen since college, and I have a BSEE. Once the FCC reduced the number of license classes and the Advanced and Extra written question pools were combined the Extra written became a far more serious affair.