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

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