What next? (Whip antenna and radio question)

[dmccormack]

My order from Aircraft Spruce arrived and now I have 6' of coaxial cable, two BNC connectors (one male, one female) and a straight whip antenna (with no connector on it).

I know how to strip the coax and expose the center wire and the wire mesh outer shielding and solder. I have ground plane materal.

So, my question is this: Do I solder the coax center wire directly to the bottom of the whip and then solder the coax shielding to the ground plane? And then put the connector on the other end and hook up to the radio?

 

[gismo]

My order from Aircraft Spruce arrived and now I have 6' of coaxial cable, two BNC connectors (one male, one female) and a straight whip antenna (with no connector on it).

I know how to strip the coax and expose the center wire and the wire mesh outer shielding and solder. I have ground plane materal.

So, my question is this: Do I solder the coax center wire directly to the bottom of the whip and then solder the coax shielding to the ground plane? And then put the connector on the other end and hook up to the radio?

I'd either attach the coax shield to whatever mounts the antenna to the ground plane or to a small screw close to the antenna. The main goal is to minimize the "loop area" formed by the separate wires of the coax. It's also important to eliminate the potential for corrosion between the shield and the ground plane. I'd also prefer a crimp ring terminal on the shield rather than soldering it to anything.

You've got the rest of it (center conductor to antenna, BNC on the far end) right.

 

[dmccormack]

I'd either attach the coax shield to whatever mounts the antenna to the ground plane or to a small screw close to the antenna. The main goal is to minimize the "loop area" formed by the separate wires of the coax. It's also important to eliminate the potential for corrosion between the shield and the ground plane. I'd also prefer a crimp ring terminal on the shield rather than soldering it to anything.

You've got the rest of it (center conductor to antenna, BNC on the far end) right.

Great! Thanks so much! I'm designing the install in my head as I type numberless pages of proposal bile.

 

[dmccormack]

OK, had some free time, stopped by Home Depot and replenished my ever-shrinking tool pile (My son builds arrows for his longbow and tools tend to migrate to the shed, never to be seen again )

I've terminated both the center coax wire and the ground shielding with ring crimp-style terminals. I crimped each and then soldered to fill in the gap and ensure security.

I kept the two ends about 1 1/2" apart, and then insulated the whole mess with Liquid Electrical Tape (couldn't find RTV, but this stuff is similar).

I checked continuity -- nothing between ground and hot, so no chance of burning up my Sporty's yet.

I plan on trying the install behind the baggage area first. While there will be some signal loss/ degradation due to the steel tubing back there, I think I should get my threshold 10 mile transmit (Chief guys with the whip installed on the wing root are seeing 50+ mile xmit with a handheld).

Thanks for the help so far and I'll post the results!

 

[weirdjim]

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I plan on trying the install behind the baggage area first. While there will be some signal loss/ degradation due to the steel tubing back there, I think I should get my threshold 10 mile transmit (Chief guys with the whip installed on the wing root are seeing 50+ mile xmit with a handheld).

I would bet NOT, but then again, I've been wrong before.

Jim

 

[dmccormack]

I would bet NOT, but then again, I've been wrong before.

Jim

Conclusion based on....?

 

[weirdjim]

Conclusion based on....?

Two things...

One, google "Faraday shield".

Two, fifty-some years in the aircraft antenna game (c.f. www.rstengineering.com and look at the antennas).

An old timer once told me that a strand of limp spaghetti in a copper septic tank makes an antenna, just not a very good one, and things can only get better from there. Yours may "work" (a purely subjective word) just not very well (another subjective word).

Jim

 

[gismo]

Two things...

One, google "Faraday shield".

Two, fifty-some years in the aircraft antenna game (c.f. www.rstengineering.com and look at the antennas).

An old timer once told me that a strand of limp spaghetti in a copper septic tank makes an antenna, just not a very good one, and things can only get better from there. Yours may "work" (a purely subjective word) just not very well (another subjective word).

Jim

There's no question that a roof mounted antenna 1/4 wave vertical whip on an extensive ground plane would outperform a bent 1/4 wave inside the aircraft fuselage frame mounted on a less than ideal ground plane, but I know from experience that there will be enough signal leaking out to achieve sufficient range (8-15nm) for Dan's needs with an antenna behind the rear seat. Fortunately the structure of a Champ doesn't qualify as a good Farady shield and since a portion of the antenna will be very near the highest point in the steel cage the attenuation won't be all that significant.

 

[weirdjim]

There's no question that a roof mounted antenna 1/4 wave vertical whip on an extensive ground plane would outperform a bent 1/4 wave inside the aircraft fuselage frame mounted on a less than ideal ground plane, but I know from experience that there will be enough signal leaking out to achieve sufficient range (8-15nm) for Dan's needs with an antenna behind the rear seat. Fortunately the structure of a Champ doesn't qualify as a good Farady shield and since a portion of the antenna will be very near the highest point in the steel cage the attenuation won't be all that significant.

A. And I'm betting from "experience" that it won't get 10 miles. In some directions, perhaps. Not in all.

B. The spelling is Faraday.

C. And exactly what does the placement of the antenna at "the highest point in the steel cage" have to do with it? Other than detuning the antenna, of course. Or haven't you heard of end effect?

Jim

 

[gismo]

A. And I'm betting from "experience" that it won't get 10 miles. In some directions, perhaps. Not in all.

B. The spelling is Faraday.

C. And exactly what does the placement of the antenna at "the highest point in the steel cage" have to do with it? Other than detuning the antenna, of course. Or haven't you heard of end effect?

Jim

A. I have such a setup in my taildragger which has a steel tube frame much like a Champ and I have no trouble communicating with the tower out to 12-15 nm regardless of direction using a KX-99. And in a plane that only goes 70 Kt, that's plenty far enough.

B. Yeah I know that looked wrong after I posted it but was too lazy to fix the typo.

C. IIRC there is a difference between an antenna deep inside an open ended container and one near the outer end of the same container, in terms of the radiated pattern. Obviously if the container were a true Faraday cage this wouldn't matter but equally obvious to me is that my airframe doesn't act like a true Faraday shield since I can transmit and receive adequately (reception works out to over 30 nm).

Keep in mind we're not trying to accomplish comm with center along an IFR XC here.

 

[Palmpilot]

How large are the spaces between metal frame members, measured in wavelengths? That will play a large role in determining the extent to which the frame will act like a Faraday cage.

 

[dmccormack]

I completed the installation to the left wing root fairing last night. After doing complete multimeter checks, I rolled it out of the hangar and radio checked with ground and tower. It was slow and I know the tower controllers so I asked to taxi around a bit and test from different spots on the runway. Loud and clear everyplace except when the tail was facing directly towards the tower (in line with the fairing's longest stretch).

The only change I plan to make is to change the way the ground wire is connected to the fairing (right now there's a U terminal that the fairing screw goes through -- I'd like to instead sandwich the ground wire between the fairing and the doubler).

So... why is the signal attenuated when the antenna is in line with the ground plane to the rear, but not straight ahead...?

Anyway, a flight test had to wait as it was officially dark by the time everything was ready to go. Since I have a sinus infection, flight testing will have to wait anyway.

 

[weirdjim]

So... why is the signal attenuated when the antenna is in line with the ground plane to the rear, but not straight ahead...?

To be a true ground plane, the metal has to be (for the aircraft com band) about 24" in all directions from the base of the antenna. In a metal ship this is not difficult because SOMETHING is going to be 24 (or an odd multiple thereof) inches from the base.

You have a true isolated ground plane. Tell me, from the base of the antenna to the end of the fairing, how long is it?

As to your grounding braid, if either the center conductor of the coax or the braid going to the ground plane is much longer than an inch or two you are flirting with the RF dragons. Remember when flirting with dragons, thou art crunchy and taste good with ketchup.

Jim

 

[dmccormack]

To be a true ground plane, the metal has to be (for the aircraft com band) about 24" in all directions from the base of the antenna. In a metal ship this is not difficult because SOMETHING is going to be 24 (or an odd multiple thereof) inches from the base.

You have a true isolated ground plane. Tell me, from the base of the antenna to the end of the fairing, how long is it?

As to your grounding braid, if either the center conductor of the coax or the braid going to the ground plane is much longer than an inch or two you are flirting with the RF dragons. Remember when flirting with dragons, thou art crunchy and taste good with ketchup.

Jim

The wing root fairing is made of two strips of .025 aluminum screwed into the fuselage and the wing (covering the gap between the two). I haven't measured it so will guess it's 70" or so long, with the antenna mounted to the rear, about 20" from the trailing edge.

The center conductor of the coax is about 1" from where it's stripped to the connector on the antenna base. The shielding is about that long, which I terminated with a crimp connector reinforced with solder.

To that I attached a 4" pigtail, which was fastened to the ground plane material (the aluminum doubler, which in turn is fastened to the ground plane with two screws. The antenna mount also passes through, also acting as a connector between the doubler and the fairing).

I can probably reduce the ground pigtail to 2".

 

[Gmaper]

Somewhere in the process of refining your installation you might borrow an SWR meter and see how well you have done. If you know any amateur radio operators (hams) you are likely able to find a meter than you can use.

SWR = standing wave ratio and is a measurement of antenna performance.

 

[gismo]

Somewhere in the process of refining your installation you might borrow an SWR meter and see how well you have done. If you know any amateur radio operators (hams) you are likely able to find a meter than you can use.

That's a good idea, especially if Dan doesn't get the range he's hoping for. But it's not crucial since all handheld (and most any good panel mount) radios will tolerate a horrible mismatch without harm.

SWR = standing wave ratio and is a measurement of antenna performance.

Technically it's a measure of feedline to antenna (and to transmitter) coupling. You can get a nearly 1:1 (i.e. perfect) SWR into a dummy load which won't transmit anything but heat. But you're right, that a reasonably low SWR would indicate that the antenna is acting properly from the transmitter's perspective.

 

[dmccormack]

Update:

After a few fits and starts, I have comms!

I had to move the antenna from the rear of the wing to the top center. This allowed for a much shorter Coax length. I also used a different Coax terminal connector.

I checked all continuity several times and all was good.

Last Saturday I took my son up for his first Chief ride and tested the radio -- loud and clear transmissions to tower while 10 miles out, AWOS loud and clear 20 miles out!

All this with a Sporty's handheld I bought in 2002, a straight whip antenna, and some RG-58 coax.

Thanks for all the help!!!