HF antenna design

[MolsonB]

When building my wings, we routed a copper wire in the bottom skin as an "S" pattern. (Over 30ft) each wing. At the time, the one person recommended doing this as I want to fly over Carribean waters in the future and said I need HF.

I have the two wires coming into my tunnel now and I'm going to run them forwards to my avionic shelf. I don't have a HF com yet and won't for a few years, just trying to plan ahead.

My tunnel is only 6 inches wide, how close can the wires be to each other? Can I shield them from each other, can they cross each other if they are shielded?

I don't know anything about HF or antennas.
Thanks for your help.

 

[Aerowerx]

Not sure I understand what you mean. Are those two wires part of the antenna (an extension there of)?

I have little knowledge of HF aeronautic antennas, but I do have some experience with HF antennas in general.

AC 43.13-2B, starting on page 23, may be of interest.

I would recommend a Balun at that point and then run a coax to the avionics shelf. Depending on what HF radio you use, there may be a remote antenna tuning box that would mount there instead of the balun.

Any Amateur's (Ham radio) in your area? They can give you some hands-on advice.

 

[Angusnofangus]

I know almost nothing about HF, not being an avionics type, but we install HF systems in new aircraft. The wire in the picture that goes from the aft fuselage to the tip of the stab and then to the forward fuselage is the HF antenna.

 

[Aerowerx]

Yes, Angus. That is the most common installation I think.

I'm not sure why he embedded the antenna in the wing. It will probably work, but not as well as what you describe. You are already at a handicap in a small plane, not having enough real estate for a good antenna.

Let's see. If we assume a nominal frequency of 6 MHz for HF aviation, that means a dipole antenna of about 80 feet. If he had an aluminum plane then he could get by with only 40 feet with the plane itself being the other half. But in a composite he has to use a dipole. He mentioned '30 feet' and 'S'. I'm not sure if 30 feet is the size of the 'S' or the length of the wire in each wing. If the length of the wire in each wing, then that is close enough IMHO. If the size of the 'S', then it sounds a bit short to me.

 

[Derswede]

The most common HF antenna I have seen in a light bird is a trailing wire antenna. Spool mounted in tail, a small drag piece to hold wire straight when extended and a sliding connector to keep contact with the wire spool. With some of the new electronics used with some of the auto-tune antennas (often called screwdriver antennas), you could make it auto tune for length. Use the fuselage as your counterpoise, and feed wire out until you get a good match.

One thing you may try is one of the autotuners (LDG, etc) which may allow you to use what you have in the wings....use a 2:1 balun to start (you may need a 4:1) and see if you can use an autotuner to get it to match on what frequencies you plan to use. It would be marginal at best. You would be best served to test in flight...tough to do, but sitting on the tarmac the antenna will be affected by the ground below it.

Another possible is to mount a screwdriver mobile antenna on the bird in a way that it would be mounted horizontally...the Yaesu ATAS-120 is an autotune antenna which when matched with one of the Yaesu HF rigs, can automatically tune itself to a proper SWR match..you would have to mount it some way so that it would not interfere nor have too much drag....off of a strut or a trailing edge. Those are easily removable for times when you don't need it. The Yaesu rigs can be modified to transmit on the marine HF bands as well.

Derswede (N4ABA)

 

[BJC]

Do you realy need HF to fly in the Caribbean? Lots of people fly down the island chain, without HF.


BJC

 

[FritzW]

One option might be to just run some coax from the panel to the top of the vert stab. Then when you want an HF antenna (not at all required to fly the Caribbean BTW, but a lot of fun) just connect a simple dipole from the rudder to each wing tip.


Just for laughs I put a quick and dirty (very quick and very dirty) 20m dipole on my VP-1. I was able to work Florida from New Mexico on very low power (QRP) using Morse code (CW).

RF choke instead of a balun, Elecraft K1, J-38 straight key ...and lots of blue tape

...like I said, quick and dirty :gig:


73 NM5DX

 

[Derswede]

Here is a good discussion....https://www.backcountrypilot.org/forum/hf-radio-5076

Derswede

 

[MolsonB]

Thank you everyone. That FAA document will take me a few weeks to get through. Good link.

I don't know too much about HF, but while I was building the wing, we threw the wire in anyways for future use. I had to look in my builders log, so the total length of wire in each wing is 40 feet. My question was more or less how close can the wires get while they run towards the avionics shelf. I changed this around this afternoon and pulled the wire out of the center tunnel and glassed them on the outside of the tunnel. Good enough for now, they are out of the way and secure.

Thanks again !

 

[Derswede]

When building my wings, we routed a copper wire in the bottom skin as an "S" pattern. (Over 30ft) each wing.

My tunnel is only 6 inches wide, how close can the wires be to each other? Can I shield them from each other, can they cross each other if they are shielded?

View attachment 58395 View attachment 58396

I tried to model the antenna with some software I have and it gave very mixed results. Tho, at your center termination in your avionics shelf, you will need to have the antenna free and clear from that area...otherwise you will have quite a bit of RF power bouncing around that area. Any antenna needs to be as "free and clear" as possible. Keep it away from metal areas as depending on the input power and resonance of the area, several hundred volts can be present on the antenna, so insulate the wire anytime it gets near any metal edge, even a little 1/8" plastic tube will help. The center wires will most likely be terminated into a balun which will also give a normal RF connection for the radio. See if you can find someone with an "antenna analyzer". Most hams have one. As the wings are apparently already covered, that eliminates the "cut and tune" method of antenna tuning. As mentioned, there are several autotuners available, some will drive a wire antenna. One leg will connect to the RF out and the other leg will connect to the frame of the autotuner, making a simple dipole. The auto tuner will "force match" the radio to the antenna. Not the most efficient, but it should work. I will see who has a tuner that can handle it. I do know that Icom has such a tuner (IC4?), and LDG has this reference... http://www.ldgelectronics.com/c/284/frequently-asked-questions.


Derswede

 

[fredoyster]

Please do not attempt to use any of the approaches suggested so far for an hf antenna on a composite aircraft. For those who would model, note that you will be dealing with a very short antenna for operation in 4 to 9 MHz generally, fed from an automatic antenna tuner. Frequency agility is a must. Voltages on the antenna will reach several kilovolts as it is brought to a very narrow band resonance with the tuner. My experience is limited to hf antennas on slightly larger planes (Navajo, Queen Air) but even then we were dealing with large insulators to allow operation without arcing.

 

[Derswede]

See my warnings above in my last post. I will disagree with Fred a bit, as most radios are going to be in the 100w range, which have automatic SWR protection, reducing power if the SWR gets high (Poor antenna, shorts/arcs, etc). However, Fred is 100% right about the voltages present. My comment about avoiding metal near the antenna is ESPECIALLY important around things like fuel tanks, etc. Even a 10W radio can generate enough RF to give a nasty burn (don't ask how I know!) and could over time, compromise a metal item like a tank. As I worked for many years designing equipment to move gasoline around, we would test the units to see how a situation could cause an explosion or a fire. Some of those videos are scary. Static electricity is enough to set off fuel vapor. Having a slow drip from a wing tank and then having an RF spark would cause someone to have a VERY bad day. I've blown up enough gear to have a healthy respect for such things. As mentioned, a trailing wire is common, and easy to tune. Lots of aircraft have used them and it is current art, don't have to reinvent the wheel...for deployment/retract, you can use a cheap electric screwdriver to drive a plastic hub to hold the wire.

That said, the wire in the wings if run away from metal objects may work. A wire from wingtip to stabilizer to the other wingtip many give enough distance to work as well. I have tried many "compromise" antennas which can be made to work OK, but are truly marginal. There was an antenna that was marketed for an amateur radio "QRP" (low power) radio which made all types of claims how good it was. Science and math showed it to be a waste of money. I built a copy and never made a contact with it. One antenna set that would work well is the PAR end fed antenna, but you will need a 40-50 ft run of wire. It may work on the wingtip/tail/wingtip arrangement. Of course, nature is hard to fool and the more wire you can put up, the better.

The "BEST" in my guessimate is a 70-90foot long wire trailing wire, as most autotuners will handle that. Your internal wing antenna can be used for receive also, heck, you could d/f just by changing heading....! If you have the internal truly clear of all metal and no possible arc paths, and no tanks in the wings, I would at least try it. If you are planning to use such an antenna on a regular basis, I agree with Fred, forget it. The external wire will run rings around it, and be much safer for all except for the occasional bird....

Derswede/N4ABA