Assuming a VSWR of 10 to 1, reasonable for 80 meters, and assuming line
losses are negligible for open wire tuned feeders, also reasonable for a
first approximation, your line losses will be about -3 dB.

EZNEC indicates that you will also have a VSWR of about 10:1 on 40 meters.

Conclusion: you will experience about -3 dB loss [1/2 S unit] on both 75
and 40 when fed with 500 ohm line.

Deacon Dave, W1MCE
+ + +

Michael Efseaff wrote:

I am proposing putting up a flat top 75 meter dipole at a height of 48'.
I would like to use the antenna primarily on both the 75 and 40 meter bands
but not always. The antenna will have to be located almost 300 feet from
the shack so I intend to use home brew 500 ohm line made with #12 solid
copper conductors. The SWR on such a line will be quite high on both 75 and
40 and be all over the map on other bands. How about 160? My question is,
what kind of transmission line losses will be present with SWR's up to 10 to
1 or even greater? I have my trusty Johnson Viking Kilowatt Matchbox to do
the tuning in the shack.
My present antenna is a 60' flat top dipole over the shack at an average
height of 34'. There is approx. 70' of 450 ohm feed line going to the
tuner. The Match Box loads the antenna on all bands from 75 thru 10 meters
with excellent results and good feed line balance. I had to modify the
Match Box to get it to 75 by utilizing the entire link and adding series
tuning to it. Eznec reports an swr of over 100 to 1 on such an antenna on
75 but it gets out well. The #18 conductors have got to have hot spots with
my 200 watts of output power! So far I haven't set anything on fire!

Comments Please, Mike K6JSY