I've always been a little worried about feed line loss in my
portable antenna / 20 meter Endfed Zepp.
TV twin lead does have more loss then lots of other feed lines that may
be used but it is light and we're talking about less than 15 feet of the
stuff here. Estimates I've found on the web show .5 - 1 dB loss per 100'
for dry, clean, matched TV twin lead. The loss for 15' then comes out
around .1 dB. Since my intended use of this antenna is temporary
QRP-in-th-Park sort of operating, keeping the twin lead clean and dry
should not be a problem. If it's raining, I just QRT, packup and go home.
Loss due to mismatch at the antenna is the other issue. See K5DVW's Nov
2006 QST article posted at
http://www.arrl.org/files/file/Technology/tis/info/pdf/q1106037.pdf
Assuming that the EFHW wire presents a 3000 ohm load to the 300 ohm
feedline then the SWR is about 10:1. Extrapolating off the left edge of
K5DVW's figure 1 chart leads me to conclude that this 10:1 feedline
mismatch adds another .5 dB loss for about.6 dB total. Feedline loss for
30' of RG-58 (about the most I'd want to carry any distance) feeding a
matched dipole at 14MHz is .4-.5 dB.
It appears that the feedline loss difference between 15' of twin lead feeding an EFHW
wire and 30' of coax feeding a dipole is negligible.
My classic 20 meter endfed zepp antenna with quarter wavelength feed
does require a tuner. While in theory the feedline could be trimmed and
stubbed to present a 50 ohm load to the transmitter (see
http://www.mfjenterprises.com/antennatalk8.php ) it is sensitive to
antenna configuration and adjacent objects. A tuner of some sort is
required to take care of the variable mismatch, but, since it is close
to 50 ohms, the tuner does not need to be "wide range".
So based on my observations for this antenna -
- Some sort of tuner is required (built into many QRP rigs these days)
- Feedline loss is essentially same as coax
- Allows flexible deployment (Vee, L, vertical, sloper)
- No ground (or radial or conterpoise) requirement
- Light weight
- Entire antenna system is off the ground
- Usually requires only one support
- Optimized for one band but usually can be tuned as a random length end
wire for other bands