"MD" wrote in message
...
I haven't got the room for a good 80m antenna. My vertical covers 40 so no
trouble there. I am looking for ideas on a short 80m that will give half
decent results. I have about 66' available straight or can make some kind
of
inverted vee ???

Put up 66 ft of wire, or any length that is convenient. Wire length is
unimportant. Inverted vee is ok. Feed it with open wire line (Or 450 ohm
ladder line) an any point you like. It will work just the same as a full
sized dipole. Do not use any coax in the feedline, losses can be very high,
even with very short lengths. You do need a well designed tuner, with low
losses. Fairly high voltages will be present at the output of the tuner.

Regards.

Frank

Put up 66 ft of wire, or any length that is convenient. Wire length is
unimportant. Inverted vee is ok. Feed it with open wire line (Or 450 ohm
ladder line) an any point you like. It will work just the same as a full
sized dipole. Do not use any coax in the feedline, losses can be very
high, even with very short lengths. You do need a well designed tuner,
with low losses. Fairly high voltages will be present at the output of
the tuner.

Regards.

Frank

As an example on 3.8 MHz: 66 ft of #14 AWG wire, fed in the center, 30 ft
high above average ground Er = 13 Sigma = 5 mS/m. Radiation efficiency
96%. Fed with 50 ft of 600 ohm open wire line: input impedance = 748 +
j2087 maximum voltage at feedpoint, with 1.5 kW input, 3.05 kV. Series C,
shunt L tuner, with inductor loaded Q of 200. Tuner Loss = 0.24 dB.

Frank

As an example on 3.8 MHz: 66 ft of #14 AWG wire, fed in the center, 30 ft
high above average ground Er = 13 Sigma = 5 mS/m. Radiation efficiency
96%. Fed with 50 ft of 600 ohm open wire line: input impedance = 748 +
j2087 maximum voltage at feedpoint, with 1.5 kW input, 3.05 kV. Series
C, shunt L tuner, with inductor loaded Q of 200. Tuner Loss = 0.24 dB.

Frank

Forgot to mention. The transmission line loss is about 0.2 dB.

Frank wrote:
Put up 66 ft of wire, or any length that is convenient. Wire length is
unimportant. Inverted vee is ok. Feed it with open wire line (Or 450 ohm
ladder line) an any point you like. It will work just the same as a full
sized dipole.

EZNEC says that a 66 ft dipole used on 3.8 MHz, fed with 450 ohm
ladder-line, will have an SWR of greater than 100:1. This can
lead to all sorts of undesirable effects including an almost
impossible to match impedance at the tuner. A practical rule of
thumb might be in order, e.g. mine = no more than 20:1 SWR on the
ladder-line.
--
73, Cecil http://www.qsl.net/w5dxp

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EZNEC says that a 66 ft dipole used on 3.8 MHz, fed with 450 ohm
ladder-line, will have an SWR of greater than 100:1. This can
lead to all sorts of undesirable effects including an almost
impossible to match impedance at the tuner. A practical rule of
thumb might be in order, e.g. mine = no more than 20:1 SWR on the
ladder-line.

Fact is Cecil I never pay any attention to VSWR, just complex numbers.
Anyway your comments made me re-analyze the problem, and I realize I made an
error in the transmission line and antenna tuner analysis. I thought the
transmission line loss I came up with was a bit low. The final numbers are
shown below. See if you agree with me; then explain where I went wrong, and
why the match will not work.

66 ft dipole, 30 ft high, #14 AWG: Input Z = 11.3 - j961 ohms.
50 ft of 600 ohm line: Input Z = 5.48 + j189.85 ohms, loss = 1.95 dB
Matching network: Shunt C = 296 pF, Series L = 22.8 uH.
(Obviously half the L for each side of a balanced.line.)
Max line voltage: 1.5 kW in = 3 kV.
Tuner loss: 0.44 dB.

Incidentally have you ever looked at a typical airport NDB site? For
example a 45 ft monopole on 350 kHz has an input impedance of 0.2 - j7054,
which is a VSWR of 4.9 million : 1 -- whatever that means -- virtually
touching the edge of the Smith Chart. I have even seen 5 mile approach NDBs
with 30 ft monopoles. Marine installations for 400 to 500 kHz operation
frequently had electrically very small inverted "L" antennas. Of course 5
S/m sure helped, but the losses in the tuners must have been significant.

73,

Frank