On 08/13/2010 06:20 PM, Owen Duffy wrote:
Alejandro wrote in
:

On 08/03/2010 04:21 PM, Wimpie wrote:
On 3 ago, 04:08, Alejandro wrote:
I would like to know if I can use RG-6 75 ohm coaxil TV cable to
feed a 14 Mhz dipole with 100 watts PEP.

That should be fine. See my notes at
http://vk1od.net/transmissionline/RG6/index.htm . See Fig 2, RG6 should
handle over 1kW continuous at 14MHz at unity VSWR, over 500W at 2:1
VSWR.

The article also shows a BNC connector option which IMHO is superior to
F connectors.

...
I also plan to use the 75 ohm cable to feed a vertical for 20 meters
32º high, that is to say aprox. 22 feet.

With that high, the impedance is 100 + i145 ohms. The reactance is
inductive and I will tune it out with a 80 pf capacitor in series made
with the same cable.

The feed line will be 1 +1/4 lambda so at the input of the line the
impedance will be (75 x75)/108 = 52 ohms

An alternative method of matching is to shorten the vertical a little
from a quarter wave, and use a shunt s/c stub. See
http://vk1od.net/transmissionline/QWVmatch/QWVmatch.htm for a
description of this and another method of matching.

The idea is to increase the radiation resistance, not to decrease it, so
has to increase efficiency.


The 8 radials will be 4 feet long with loses of aprox 5 ohms, so
efficiency will be near 94%.

This isn't entirely clear to me.

Owen

If I define radiation efficiency as:

Rr/(Rr+Rl)

where Rr; radiation resistance
and Rl: loss resistance

and take Rl=5 ohms for 8 radials of 8 feet each

For a 25º length radiator: 36/(36+5) = 88%
For a 32º length radiator: 100/(100+5) = 95%

Losses are reduced from 12% to 5% by only adding 5 feet to the 20 meter
vertical quarter wave antenna and with a 75 ohm coax of 1,25 wavelength
I can get a perfect match to a 50 ohm output impedance transmitter.

Alejandro Lieber
Rosario Argentina


Real-Time F2-Layer Critical Frequency Map foF2 at:

http://1fcr.com.ar

--- news://freenews.netfront.net/ - complaints: ---

Alejandro Lieber wrote in news:i4h24t$1rl0$1
@adenine.netfront.net:

....
and take Rl=5 ohms for 8 radials of 8 feet each

That specifically escapes me.

Owen

On 08/18/2010 02:49 PM, Owen Duffy wrote:
Alejandro wrote in news:i4h24t$1rl0$1
@adenine.netfront.net:

...
and take Rl=5 ohms for 8 radials of 8 feet each

That specifically escapes me.

Owen


The 8 radial can be as short as 4 feet long for the same results.
I came to these results from R. J. Edwards (G4FGQ) program ENDFEED.EXE
It's interesting to note, that radial longer than 1.3m (4 feet), always
increase earth looses.

The complete modeling results can be seen at:

http://1fcr.com.ar/endfeed.pgn

Alejandro Lieber
Rosario Argentina
LU1FCR

http://1fcr.com.ar

--- news://freenews.netfront.net/ - complaints: ---

On 08/18/2010 06:16 PM, Alejandro Lieber wrote:
On 08/18/2010 02:49 PM, Owen Duffy wrote:
Alejandro wrote in news:i4h24t$1rl0$1
@adenine.netfront.net:

...
and take Rl=5 ohms for 8 radials of 8 feet each

That specifically escapes me.

Owen


The 8 radial can be as short as 4 feet long for the same results.
I came to these results from R. J. Edwards (G4FGQ) program ENDFEED.EXE
It's interesting to note, that radial longer than 1.3m (4 feet), always
increase earth looses.

The complete modeling results can be seen at:

http://1fcr.com.ar/endfeed.pgn

Alejandro Lieber
Rosario Argentina
LU1FCR

http://1fcr.com.ar

--- news://freenews.netfront.net/ - complaints: ---

Sorry:

the link to the results is:

http://lieber.com.ar/enfeed.png

Alejandro Lieber

--- news://freenews.netfront.net/ - complaints: ---

On Wed, 18 Aug 2010 18:21:08 -0300, Alejandro Lieber
wrote:

the link to the results is:

http://lieber.com.ar/enfeed.png

Not Found
The requested URL /enfeed.png was not found on this server.

Always use cut/paste from a browser actually viewing the page you wish
to share with others.

73's
Richard Clark, KB7QHC

On Wed, 18 Aug 2010 19:45:37 -0700, Richard Clark
wrote:

On Wed, 18 Aug 2010 18:21:08 -0300, Alejandro Lieber
wrote:

the link to the results is:

http://lieber.com.ar/enfeed.png

Not Found
The requested URL /enfeed.png was not found on this server.

Likewise I get: " Sorry, the page you were looking for could not be
found"

73

Roger (K8RI)


Always use cut/paste from a browser actually viewing the page you wish
to share with others.

73's
Richard Clark, KB7QHC

Roger wrote in
:

On Wed, 18 Aug 2010 19:45:37 -0700, Richard Clark
wrote:

On Wed, 18 Aug 2010 18:21:08 -0300, Alejandro Lieber
wrote:

the link to the results is:

http://lieber.com.ar/enfeed.png

Not Found
The requested URL /enfeed.png was not found on this server.

Likewise I get: " Sorry, the page you were looking for could not be
found"

And when I click it in Roger's quote of the original post, it works for me.
Why would that be?

Owen

On Aug 18, 2:21*pm, Alejandro Lieber wrote:
....
Sorry:

the link to the results is:

http://lieber.com.ar/enfeed.png

For what it's worth, the link works fine for me.

Cheers,
Tom

Alejandro Lieber wrote in
:

....
The 8 radial can be as short as 4 feet long for the same results.
I came to these results from R. J. Edwards (G4FGQ) program
ENDFEED.EXE It's interesting to note, that radial longer than 1.3m (4
feet), always increase earth looses.

The model sensitivity to radial length suggests that the radials are
modelled as short transmission line immersed in a lossy medium, in which
case, the assumptions about soil conductivity and permittivity are very
important to the model accuracy. It is unlikely though that shallow
buried radials are very consistent in behaviour due to the effects of
rainfall.

It would be interesting to measure the antenna and compare it with your
model predictions.

Owen