I'm trying to figure out an old handmade antenna I picked up at a swap meet a few years back. It is a dipole (or "doublet"?) that is both made of coax but also fed by a length of built in coax.
For the elements, it looks like someone has taken a single length of coax approximately 94 feet long and left its center conductor only as (?) a physical support. In the middle of this length, the coax shield was uncovered and a small section cut out, splitting the outer conductor into two separate elements.
This seems like an odd length — as a dipole this might be resonant somewhere in the ballpark of 4.98 MHz, at least using the
468 / frequency rule of thumb. That's getting towards the new 60m band, but there's a 1981 date written on the antenna, I'm assuming to commemorate its construction — long before the 60m band became available in the USA.
So I'm guessing instead this was meant to be some sort of multiband compromise antenna?
In fact the "Five Bands, No Tuner" G0FAH antenna described within e.g. http://n7twt.net/Antennas/9506059%20Five%20Bands,%20No%20Tuner.pdf (labelled "June 1995 QST Volume 79, Number 6") and also found on page 32 of http://www.hamclass.net/ranv/pres/HC16MultAnt.pdf seems like one possible candidate — except…
Whereas the G0FAH and most other "wrong sized" doublets seem to be fed with balanced 450-Ω ladder line, the antenna I "inherited" was fed with unbalanced 50Ω coax soldered directly on the feedpoint! [Also the G0FAH was apparently published 1995 but this antenna might have been built in 1981 already…]
[In the picture above, I've already disassembled it for troubleshooting, and chopped the end of the feedline clean. Originally it connected via each of the two solder blobs remaining on the lower coax.]
Is this any sort of known antenna design? Might its construction be improved by at least a balun/choke at the feedpoint, or is it maybe a lost cause without the proper length of balanced line to serve as a transformer section?
Some more precise lengths in case it's useful:
- each leg of the dipole reads as 14.26m at 70% velocity factor
- the feedline coax read as 6.23m, again assuming 70% velocity factor
(The coax is labelled as 58/U which I see is spec'ed to be 66% VF, so the numbers above might be a bit off of the physical length.)
Here's a quick-and-dirty VNA view of the antenna, before I took it apart, seen through the built-in feedline. Note that the trace was "flakey" due to what I think was a short within the feedpoint — the resonant dips would appear and disappear with the breeze: