On 18 May 2005 00:52:28 -0700, "Cliff" wrote:

Just wondering where there is the 'lacking of good explanation' areas?
Shoot, I thought it was a fairly well done site! Much better than most
of us can truly understand. I wished that when I built the first
antenna I had had acess to that site. Would have kept a lot of 'trial
and error' work from going on.


*****
Overall it is a very good site. It is by far better than many that I
have seen in the past. But when some try to explain transmission line
issues with a small working knowldge of them, the all to often too
simple explanations or are lacking or misleading.

In the explanation of why coax affects SWR. While it is not bad, it is
typically and over symplistic. The real problem is that comon mode
currents are often not understood as to how they are generated and
what effects they have on SWR meters. Another thing is that the SWR
bridges made for CB and many for Ham use are poor instruments to
measure SWR. They are only good to about 3:1 and after that their
accuracy suffers greatly. The best way to measure the SWR of the
antenna is with a dual directional coupler. Preferably the power
measurements should be a spectrum analzer.

Also coax impeadance is determined by ratio of the inside diameter of
the outer conductor to the outside diamter of the inner conductor and
teh dielctric constant of the medium in between. Not just solely the
ratio of the diameter. That would be true only of air dielectric coax.

Coax loss is solely determined by the dielectric and not the shield.
As long as the shield is in relatively good shape the sole called
"leakage" loss is hardly measureable, even in relatively high power RF
transmissions. A shield of 80% is still nearly 100% effective even at
frequencies as low as 10 MHz. The openings in the shield would have to
be at least 1/16 of a wavelength in order to start to be able to
measure leakage. At 10M that is 1.25 meters.

What causes dielectrtic breakdown is chlorine in the PVC,
PolyvinalChloride, to leach into the dielectric and cause the
characteristic impeadance to change. Also chlorine from sea spray that
can leak into the coax from poorly assembled coax connectors. The site
is right in that cox does need to be changed. Even the best
installations require changing at least once every ten to fifteen yrs.

One thing also about antenna design and installations. Unless you
model the ground (Earth) for your area, you have missed a major
influience on the performance of any antenna design or installation.
There is a deal of hunt and peck to tame some designs. Especially
dipoles that are not at least a wavelength above ground. Some designs
behave better than others.


james

james wrote:
On 9 May 2005 17:47:14 -0700, "Cliff" wrote:

When compared to a stated db in a ground plane you should get at
least
10db better with a 2 element quard. Go to:
http://www.signalengineering.com/ultimate/
*****

I perused this page and for most of the info is adequate
explaination.
I think he really lacks good explanation is some areas.

james