VHF Airband mod to RS and Uniden ducks

[nanZor]

Most Radio Shack and Uniden oem rubber ducks end up in the drawer. How about putting it to use as tuned airband whip?

This mod is nothing more than tightly wrapping about 21 inches of wire alongside the duck, and for eye-safety, wrapping a little loop at the end.

This mod relies upon capacitive coupling to the original helical whip, and while good for airband, is fairly narrow, so it is NOT a good substitute for other vhf services above airband, even if you try to cut it down further.

Here's a shot of the bottom of the duck, with the #12 house wiring taped alongside. Blue painter's tape was used so there would be no messy electrical tape goo left behind when it is removed. Leave a little space between the top of the bnc connector and the wire. Pictured is a Radio Shack oem duck easily identified with the two grooves. I started the wire just above the two grooves.

The length is not exactly critical as anywhere from 20 to 21 inches will do, but don't stray too far beyond this.

 

[nanZor]

Here is the overall shot. The antenna at this point is pretty stiff, so you REALLY want to make sure the radio doesn't topple over since now the whole thing is very rigid, and there is the chance that you can break the bnc connector or separate the connecting wire inside.

 

[nanZor]

SAFETY FIRST - this is the single most important part of the mod. I know I sound like your shop-teacher, but you do not want to lose your eyesight or endanger others without it. Since it is not likely to be used portable, but sitting on a desk, you don't want to accidentally lean into the sharp tip.

It takes ONE SECOND with needle nose pliers to be safer.

 

[nanZor]

How does it perform?

Again, this is meant only for vhf airband. Trying to tune the whip for anything above 135 mhz is very inefficient due to the LC-ratio of the duck and wire. The Radio Shack / GRE bnc ducks are actually a little better than the Uniden BNC ducks, since the RS ducks already have a lower resonant frequency than the Uniden's.

This is also applicable to the sma-style Uniden ducks.

A Comet CAA-500 swr-analyzer reassured me that this was working, but swr isn't the only story. I can make a paper-clip have a good swr, but that doesn't make it efficient!

I also used the S-meter from an Icom R20 which has higher resolution than the usual 4-5 bar s-meter found on many scanners.

Real-world testing reveals that this setup, ON AIRBAND, is pretty much the equivalent of a properly adjusted telescopic whip or even the Maldol AL-500H. It is also in fact just a tad better than the venerable Diamond RH-77CA, but not by much. Of course it works better than the oem ducks.

Lowband monitoring will be improved over the oem duck, but then again, we are so inefficient on lowband, that nearly any small improvement will be noticeable, even if the antenna is still way off tuned. The same goes for vhf and uhf monitoring other than airband - it will be pot-luck whether or not it is an improvement over the oem duck, so judge accordingly.

This quick and simple mod serves well in an emergency, or if you want to see if investing in a quality telescopic would be worth it or not. Again, this trick is primarily designed for airband monitoring. Inexpensive scanners like the RS Pro-404 should benefit from it now being a resonant whip, and somewhat narrow / hi-q due to the capacitive coupling reactance - unless you are just getting hammered on 2 meter images.

Just be careful not to let the scanner fall as the bnc connector could be damaged.

 

[Rt169Radio]

Interesting idea.

 

[nanZor]

This mod is nothing more than tightly wrapping about 21 inches of wire alongside the duck, and for eye-safety, wrapping a little loop at the end.

Ooops. Correction - there is only a straight wire underneath the tape - no wrapping involved except for the eye-protection loop at the end. I suppose one could substitute tie-wraps instead of the tape.

At first, I thought I could just trim the wire to bring it into resonance anywhere on vhf, but quickly found out that the wire drops the helical coil's own resonant frequency and limits the bandwidth to about 3:1 peak swr at the 118 to 135 mhz band edges. Elsewhere, even when trying to size the whip for other bands, the reactance was just too much for my tastes, thus I only advise serious testing on airband and anything else is pot-luck.

Frankly, I was not expecting much, but was surprised that it worked as well as it does. The RS and 2-meter amateur ducks that are rigid seem to drop right into place. The Uniden ducks just barely make it.

 

[mule1075]

Just tried this and have to say it works quite well.Thanks for posting.

 

[Iffy]

I suppose one could substitute tie-wraps instead of the tape.

Ummm, how 'bout some heat shrink? Just don't heat it too 'tight'.

 

[nanZor]

That would work too. I cut open a RS oem duck, and aside from just being a helically wound coil, it has a stiff plastic insert inside the coil which makes it better than the Uniden duck for this application. Although placing a dielectric inside a coil makes it even more lossy than it already is...