Eureka -- I think I finally found the reasons for the high VSWR on my 300 ohm feed line.
I’m writing it all down in case someone else runs into the same problems. The bottom line is -- be careful using shielded twin lead!
As Mike, AD1OS said, “I didn’t know my antenna was bad until SWR bridges were invented”.
The antenna, a folded dipole, seemed to work just fine. But the VSWR(300) measured a huge 6 : 1. Since 7 MHz losses are low on TV twin lead even at this SWR, I seemed to able to operate OK. But obviously something was wrong.
1.) The first thing was to adjust the antenna length to try to minimize the SWR.
Increasing the length from 64’ 3” to 67’ 3” got the VSWR(300) down to 3.6 : 1 at my operating frequency - but it wouldn’t go any lower.
2.) The next thing was to scrap the idea of having the folded dipole wires cable-tied together. (I was trying to simplify the construction.)
G3TXQ’s paper convinced me that the very close spacing may be causing problems. So, I put 2” spacers between the wires. The antenna now looks like a classic folded dipole.
The VSWR dropped, and after another round of length adjustments - increasing it to 67’ 7” - I got the VSWR(300) down to a minimum of 2.8 : 1. But again, it wouldn’t go any lower. Something else was wrong in addition to the antenna construction.
3.) So, I decided to separately evaluate the 3 pieces of 300 ohm twin lead in the feed line system. The 3 pieces were:
a. - A 4’ section of cheap, clear TV twin lead going from the 7 MHz rig to an indoor junction.
b. - A 15’ piece of quality shielded TV twin lead that went from the basement, through the wall to an outside junction.
c. - A ~ 36’ section of cheap brown plastic covered TV twin lead that went from the outside junction up to the antenna.
The evaluation technique I used - right or wrong - was:
i.) - Measure the VSWR(300) of the piece of twin lead open circuited at the far end.
ON4UN - John Devoldere - says in his book “Low Band Dxing”, page 6-2, that cable attenuation can be measured this way by calculating: dB loss = log[(swr + 1)/(swr - 1)].
I got roughly ~ 0.2 to 0.3 dB/ 100’ for the pieces, which is about right at 7 MHz according to the ARRL Handbook and John’s book. Hmmm, the pieces were not “lossy”.
ii.) - Measure the VSWR(300) of the pieces with various resistive loads at the end.
The idea was to see what resistive load gave the minimum VSWR(300). It should be ~ 300 ohms.
The 2 “cheapy” pieces showed a minimum VSWR(300) of 1 : 1 at a ~300 ohm load. But the “quality” shielded piece of twin lead had a minimum VSWR(300) of 2 : 1 at a ~100 ohm load.
iii.) - Measure the twin lead impedance Zo = sqrt(L/ C), where C of the piece is measured with the far end open circuited, and L is measured with the far end shorted. I used my AADE meter. The impedance's of the 3 pieces should be about 300 ohms
The 2 “cheapy” pieces had a measured value of Zo ~ 270 ohms (close enough to 300), but the “quality” shielded piece measured Zo ~ 160 ohms.
That 15’ piece of shielded twin lead is a “bad actor”.
I don’t know what effect the shield is having, and at this point I don’t care. I just replaced it with a piece of “cheapy” TV twin lead through the wall. (Maybe I’ll look into the shielding question later. Maybe.)
The 67’ 7” folded dipole now has a VSWR(300) of 1.3 : 1 - and is the minimum value that I can get. A heck of a lot better than 6:1.
I think the 1.3 : 1 ratio makes sense too.
It says that the x4 transformed antenna resistance is about (270 ohms/ 1.3) = 208 ohms. Therefore, the antenna’s radiation resistance is about (208/ 4) = 52 ohms.
And, since the average height of the antenna is only ~ 25’, ie about 0.18 wl high, a radiation resistance of somewhere around 50 - 60 ohms makes sense. See Moxon’s “ “hf antennas for all locations” book, pg 102, for example.
So, the antenna works AND the VSWR(300) is now low. Life is good.
1.) The VSWR(300) bridge is a VERY handy little gadget.
2.) Don’t take short cuts making folded dipoles
3.) Be careful using shielded twin lead.
Sorry for being so long winded again. Fun stuff.