An antenna of this size for 75 and 40 meters has a very low radiation resistance, typically 8-15 ohms. My 75 meter radiation resistance appears
to be about 10 ohms. The lossier versions of such mobile antennas have higher impedances largely due to resistive losses. Trading high performance
and narrower bandwidth for lower losses seemed acceptable to me, but it requires some way to match the impedance to the coax, particularly when
operating slightly off resonance with a tuner. I went with the unbalanced to unbalanced transmission line transformer to accomplish this impedance
matching. Here's why:
The great advantage of transmission line transformer matching over other methods, such as tapped coils and tuned shunts, is that it's not very
sensitive to frequency. Or in other words, the matching device does not effect the resonant frequency of the antenna. These devices can be
incredibly efficient when built with the right materials. Loss measurements are typically less than 0.1dB. My crude tests in the shack show
better than 98% efficiency, somewhat better then 0.08dB loss! Sure beats the loss on the coax with a 4:1 impedance mismatch.
Once your antenna is tuned to resonance you could remove the xfmr and still operate with reasonable effectivness since the tuning won't channge.
Yes, your SWR will rise but your resonant antenna currents will still be where you need them to radiate effectivley. If using an antenna tuner
without the transformer, you can certainly tune the "system" for a match, but the losses after the tuner remain the same since the cable and load
are still mismatched. Similarly, you can retune, or change bands (40 meters in my case) and not have to play with the matching network at all
since the transformer is basically frequency independant throughout the HF band. Different types of cores can be selected to optimize the passband
for different frequency ranges. Some are best from several hundred KHz through 10 MHz, while others will be most effective from a few MHz to over
50MHz or more. My xfmr falls in the latter category as I assumed 160 Meters to be a low priority, and likely an even lower radiation resistance
will require a differnt xfmr for that band anyway.
Input impedance on this antenna on 40 meters is about 17 ohms, so the transformer still does a good job matching down from 50.
To be fair, I did play around with some conventional methods of shunt coil and series capacitor matching, but it changed the resonance of the
whole antenna and I couldn't change bands without having several prefab matching networks to swap. Way too much work, certainly nothing I could
jump out and do at a red light.
75m bandwidth is 9 KHz and 40m bandwidth is 16 KHz on this antenna. If you don't have a tuner in your radio you can operate reasonably well over
this bandwidth without it. After that you have to move or add coil taps (hence the convenience of my bandswitch).
Construction:
The mount uses a standard insulated SO-239 to 3/8'-24tpi whip coupling through the bracket. $4.99 at Radio Shack. This allowed me to fabricate a
sealed enclosure using 2-1/2" PVC pipe with panel mount SO-239s on each end in polycarbonate and caps. End caps are sealed with O-rings and held by three 4-40 screws.
Finally, a 26 gauge copper strap from end to end provides a solid ground for input and output and is stamped with each terminal's impedance.
The 2 1/4" overall diameter of the transformer is fits perfect in this pipe.
Basically, this is a simple unbalanced-unbalanced 4:1 Ruthroff transformer. It uses two toroidal cores (although one would have been
sufficient) of medim permeability type 43 ferrite (mu = 850). Around this is wound 9 turns of DOUBLED 50 ohm tranmission line. Ideally, this line's
impedance should be the arithmetic mean between to the two terminating impedances. In this case, matching 50:12.5 ohms would require 25 ohm coax or
stripline. Well, I didn't have either, and I didn't want make 25 ohm stripline, for reasons I won't go into. But I did have some of that nice 50 ohm,
teflon insulated RG-174U. It's about 0.1" diameter and using two in parallel yeilds 25 ohms. I don't know why but the idea of using parallel windings
for this purpose isn't suggested in the references I've reviewed. So I take credit for assuming the obvious. I later discovered others have done this
successfully. It works great!
