Having trouble responding to the above posting. Hope it is not duplicating
what I have tried to send before.

The nominal impedance of a 40 m dipole on 20 m, at 30 ft above an average
ground, is
4700 + j0. There may be some inductive or capacitive reactance present --
depending on the exact length of the antenna -- but it will not effect the
transmission line losses significantly.

100 ft of RG 58 exhibits a total line loss of about 13 dB when terminated
with the above impedance. i.e. 100 W in gives 5 W radiated.

For the above analysis I used NEC 2 for the antenna model, and ARRL's TLA
for the transmission line loss.

The results are very similar to K6MHE, but should show zero reactance on
some frequency. Anyway the losses are close.

Regards,

Frank

Frank wrote:
The nominal impedance of a 40 m dipole on 20 m, at 30 ft above an average
ground, is
4700 + j0. There may be some inductive or capacitive reactance present --
depending on the exact length of the antenna -- but it will not effect the
transmission line losses significantly.

100 ft of RG 58 exhibits a total line loss of about 13 dB when terminated
with the above impedance. i.e. 100 W in gives 5 W radiated.

While 1/4WL (or 3/4WL or 5/4WL) of 450 ohm ladder-line yields an
impedance at the transmitter of 43 ohms for a 50 ohm SWR of 1.16:1
without a tuner and very low line losses. What could be sweeter?
--
73, Cecil http://www.qsl.net/w5dxp

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"Cecil Moore" wrote in message
...
Frank wrote:
The nominal impedance of a 40 m dipole on 20 m, at 30 ft above an average
ground, is
4700 + j0. There may be some inductive or capacitive reactance
present --
depending on the exact length of the antenna -- but it will not effect
the
transmission line losses significantly.

100 ft of RG 58 exhibits a total line loss of about 13 dB when terminated
with the above impedance. i.e. 100 W in gives 5 W radiated.

While 1/4WL (or 3/4WL or 5/4WL) of 450 ohm ladder-line yields an
impedance at the transmitter of 43 ohms for a 50 ohm SWR of 1.16:1
without a tuner and very low line losses. What could be sweeter?
--
73, Cecil http://www.qsl.net/w5dxp


I agree, but many people do not seem to get it.

73,

Frank

"Cecil Moore" wrote in message
...
Frank wrote:
The nominal impedance of a 40 m dipole on 20 m, at 30 ft above
an average ground, is 4700 + j0. There may be some inductive
or capacitive reactance present --depending on the exact length
of the antenna -- but it will not effect the transmission line losses
significantly.

100 ft of RG 58 exhibits a total line loss of about 13 dB when
terminated with the above impedance. i.e. 100 W in gives 5 W
radiated.

This seems a bit academic. I don't know anyone goofy enough to
run 100 feet of RG-58 with 100:1 SWR; do you?

While 1/4WL (or 3/4WL or 5/4WL) of 450 ohm ladder-line yields an
impedance at the transmitter of 43 ohms for a 50 ohm SWR of 1.16:1
without a tuner and very low line losses. What could be sweeter?

What could be sweeter? I'd say a 20m dipole, coax fed. (Mine shows
1.0 SWR at band center, and under 1.5 full-band. Not fussy about feedline
length. Feedline also runs through a metal conduit, with no problems.
No tuner of ANY kind needed, distributed or conventional. Minimal
loss. QRO FB.)

But if one MUST operate a doublet far from 1/2-wave resonance, then
a balanced feedline plus tuner is the way to go. And implementing the
tuner in distributed form by tuning feedline length has some advantages
over a conventional, lumped-constant tuner. To my mind, the MAIN
advantage of the tunable feedline is that it can handle high power at
low cost. I'm a bit surprised you don't talk that advantage up more.

73, Ed, W6LOL

--
73, Cecil http://www.qsl.net/w5dxp

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Old Ed wrote:
What could be sweeter? I'd say a 20m dipole, coax fed.

Well, a 66 ft. dipole used on 20m does exhibit gain over a
33 ft. dipole used on 20m by almost 2 dB.

To my mind, the MAIN
advantage of the tunable feedline is that it can handle high power at
low cost. I'm a bit surprised you don't talk that advantage up more.

Funny thing about that. I came up with that idea when I bought
a 500 watt amp and didn't want to have to buy a 500 watt tuner.
--
73, Cecil http://www.qsl.net/w5dxp

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"Frank" wrote in message
news:JFqqe.44797$on1.7535@clgrps13...

"Cecil Moore" wrote in message
...
Frank wrote:
The nominal impedance of a 40 m dipole on 20 m, at 30 ft above an
average
ground, is
4700 + j0. There may be some inductive or capacitive reactance
present --
depending on the exact length of the antenna -- but it will not
effect
the
transmission line losses significantly.

100 ft of RG 58 exhibits a total line loss of about 13 dB when
terminated
with the above impedance. i.e. 100 W in gives 5 W radiated.

While 1/4WL (or 3/4WL or 5/4WL) of 450 ohm ladder-line yields an
impedance at the transmitter of 43 ohms for a 50 ohm SWR of 1.16:1
without a tuner and very low line losses. What could be sweeter?
--
73, Cecil http://www.qsl.net/w5dxp


I agree, but many people do not seem to get it.

73,

Frank

=======================================

Just out of curiosity I calculated the following details of 100 feet
of RG-58 terminated with 4700 ohms at 14 MHz.

Line input impedance = 12.3 - j37.7 ohms.
SWR vs 50 ohms at termination = 88.6
SWR vs 50 ohms at generator end = 6.35
Matched loss of 100 feet of line = 1.3 dB
Actual loss = 11.4 dB
With 100 watts input, output into 4700 ohms = 7.2 watts.

A tuner is essential. There will be additional loss.

By the way, at the first current loop, line current is 3.41 amps. As
the inner conductor diameter of RG-58 is only 0.78 mm, the temperature
of the pvc jacket can rise to about 20 degrees C above ambient. So
don't hold the line in the hand for too long.
----
Reg.

Cecil Moore wrote:
Old Ed wrote:

What could be sweeter? I'd say a 20m dipole, coax fed.


Well, a 66 ft. dipole used on 20m does exhibit gain over a
33 ft. dipole used on 20m by almost 2 dB.
. . .

Most people can't or don't rotate a 66 foot dipole, and it has that much
gain over a dipole half the length only in two specific directions. It
has a gain of one dB or more over the shorter dipole only over 60 of the
360 degrees of the compass; and for 280 out of 360 degrees, it has
*less* gain than the shorter dipole -- in fact as much as 14 dB less.
For most people the longer dipole is a poor choice unless there's a
specific narrow direction you want to favor at the expense of most other
directions.

You lose a lot of information when you reduce an antenna pattern to a
single "gain" number, just as you do in reducing complex data to an
average. Keep in mind the statistician who drowned while crossing a
creek whose average depth was only three feet.

Roy Lewallen, W7EL

Roy Lewallen wrote:
Most people can't or don't rotate a 66 foot dipole, and it has that much
gain over a dipole half the length only in two specific directions.

And one of those directions is straight toward Arizona where
most of my old Intel ham buddies are. :-)
--
73, Cecil http://www.qsl.net/w5dxp

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