To Tom, K4TJD
I just had a few additional thoughts to add to my previous post. If you feed a 20 meter inverted vee with an ELECTRICAL half wavelength of 300 ohm TV twinlead the Z at the rig end of the line will be very approximately 50 ohms balanced. As I mentioned usually QRP operators can just hook this balanced line to the unbalanced output of the rig and usually get away with it. However if this idea bothers you, you could construct a simple coax choke and insert it between the twinlead and your rig. If you scramble wind 9 turns of RG-174 coax into a coil approximately 1 1/2 inches in diameter, it will have a reactance of approximately 500ohms on 14 MHZ and 200 ohms on 7MHZ. This coil will weigh much less than one ounce and give you many db of isolation even on 7 MHZ. You could use a 6 turn coil for 14 MHZ and have nearly 300 ohms of reactance with only about 1/2 oz of weight.
With this choke at the rig end of the twinlead, it would also give you the option of adding a short additional length of RG-174 or RG-58 to reach your rig. If your inverted vee is far away you could use additional multiples of 1/2 wavelength of twinlead before the choke to reach your rig. These coax choke values are just some quick measurements I made tonight and are not proposed as best designs. Perhaps a different diameter or no of turns would be better. If you want a better choke or want to operate on 80 or 160 meters, possibly you would want to wind some coax on a ferrite core. I did not measure the loss in this choke, however since the length of coax is minimal I would assume the loss to be quite small.
The above ideas (not designs) should work well at QRP levels. For higher power operation you may want a better choke (with more isolation) and coax which is able to handle more power than RG-174. Note if possible use an analyzer to cut the twinlead to an exact electrical 1/2 wavelength.
Rick KL7CW
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