1
$\begingroup$

I'm working on a kite-supported antenna for an 80m QRP rig -- to be flown at beaches, on mountaintops, and possibly other places where I want to work HF but don't want to lug poles or give up efficiency with a sub-optimal antenna. This will be a center-fed half wave dipole, so about 132 feet of wire.

I have the kite already in hand, a carbon spar nylon sail conyne (aka French war kite, a hybrid of a box and a diamond) just over three feet in length. It's stable, flies in a broad range of wind, and has good pull (so keeps the line taut).

To support the antenna wire, I plan to use 000 size locking snap-swivels (made for fishing) bent to the kite line with prusik knots. The knots can be loosened and moved when adjusting the setup, and are light enough that they won't affect the kite if I'm not lifting the antenna. Because they grip the line without forcing it into a sharp bend, they will have almost no effect on the line strength.

My question concerns the snap swivels (similar to these). These are effectively tiny wire loops that will hold the antenna, one at each end, one in the middle, and two or three spread along each leg of the dipole.

Due to their small size, I wouldn't expect these loops to have any noticeable effect on the antenna's radiation in the 80m band, but I wanted to ask to be sure. I'd hate to key up the transmitter and find out all the power I'm sending up the wire is going into heating the snap swivels. Am I correct in thinking a stainless and brass snap swivel, with the snap loop about an inch long, will have no significant effect on the radiation of a half wave dipole on 80m band?

$\endgroup$
  • $\begingroup$ How are you centre feeding it? What about end fed, so you don't have to worry about the weight and proximity of a feedline, or for that matter carrying any great lengths of coax around at all? Simple to do on 80M with just a coil and capacitor. $\endgroup$ – R Johnson Jul 4 at 5:22
  • $\begingroup$ @RJohnson It's a speaker wire antenna, fed (at present) through the 30+ feet of unsplit wire remaining on the 100 foot spool (supposed to be close to 75 ohm); I've also got 50 feet of 50 ohm coax available. I'd have to start over building an end fed, including building a matching unit and/or balun -- which reduces the portability of the rig. $\endgroup$ – Zeiss Ikon Jul 5 at 14:14
  • $\begingroup$ I used to use a end fed kite supported vertical (ish!). It was especially useful where there was a constant wind such as in coastal ares. You could see the kite for miles and couldn't loose the camp!. At night we used a small 'carrier kite on the main line to hoist a light up. Have fun experimenting!! $\endgroup$ – R Johnson Jul 5 at 15:28
  • $\begingroup$ @RJohnson This is a "sloper" dipole, strung along the kite line. Haven't managed to use it yet -- had it out for field day, but there was no useful wind (2-3 mph, variable direction, just thermal bubbles all day). I'll likely modify the setup so the feedline comes back to the lower end and an additional 50 feet of coax lets the antenna get up off the ground a bit -- but might not get to use it until next winter/spring, since the only wind we get in summer is connected to thunderstorms... $\endgroup$ – Zeiss Ikon Jul 5 at 15:33
4
$\begingroup$

Am I correct in thinking a stainless and brass snap swivel, with the snap loop about an inch long, will have no significant effect on the radiation of a half wave dipole on 80m band?

Yes. This object is over 1,500 times smaller than the wavelength; you can therefore conclude it will have much less effect than other features of your antenna system. The material and shape don't affect this conclusion.

$\endgroup$
1
$\begingroup$

I wouldn't worry about radiation pattern. I might worry about metal to metal arcing, as ends of a dipole are a high-voltage RF node adjacent to whatever static charge the plastic kite string picks up from the wind.

See: What is the peak voltage at the tips of a dipole antenna?

$\endgroup$
  • $\begingroup$ Given this is mainly intended as a portable/QRP setup, i doubt high RF voltage at the tips is a major concern. The set I'm building it for puts out about 1W on a fresh 9V battery, though I might well get one on this band that emits more at some point. Thanks for mentioning it, though. $\endgroup$ – Zeiss Ikon May 14 at 16:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.