Those "Useless" Whips

Often seen at club meetings in the “Free to good home” heap, there seems to be an inexhaustible supply of HF mobile whips, generally out of the mines in South Australia or Northwest Queensland, similar to a Terlin “Outbacker”, but of, course, custom wound to commercial HF frequencies temptingly close to amateur allocations, but no cigar.

You could, of course, break open the appropriate windings, and reduce the number of turns (assuming you want to go up, not down, in frequency), but that would almost certainly impair the the robustness and watertightness of the assembly.

I was given, for free, a pair of these Terlin clones with a tap for 6846 kHz, close to our 40 mtr allocation, but, given the fairly high Q of most helically wound mobile whips, it may as well have been the opposite end of the spectrum. My test subject actually came in closer to 6750 kHz, probably because I had misplaced the original “wander wire”, and grabbed something off the bench that was clearly too long.

One morning, my brain woke up long before it would be appropriate to get up and make noise, so it chose to apply itself to this issue. Why, I do not know. “What if I encased a portion of the 40 mtr winding in foil? It should act as a capacitively-coupled short”. The amount of foil, and indeed it's placement, would obviously have to be determined empirically (consultant-speak for “suck it and see”), since there are no nomographs for this particular brand of lunacy.

In our household at least, 30 cm is the width al-foil comes from the store, so I requisitioned enough to do a double wrap around the whip. Not knowing where to position it on the “40 mtr” winding, I started at the bottom. Woah! Instant resonance shift from 6750 kHz to 7176 kHz, and the Q looks as good as it did before my addition. Maybe I should buy a lotto ticket? Obviously, for mobile-in motion use, some heat shrink over the foil would be in order., but it is gratifying to see something that was inching toward the bin suddenly acquire a new lease on life....

John VK4TJ