"Jimmie" wrote in message
om...

"Lancer" wrote in message
ews.com...
On Thu, 04 Nov 2004 21:59:02 GMT, sideband wrote:

tnom:

Explain this, then... CB radio in my Peterbilt, with a mirror mounted
Hustler HQ-27 on the driver's side.. 6 feet of coax shows a 1:1 on 19,
1.1:1 on 1, and 1.1:1 on 40... 12 feet of coax shows the same
readings... 18 feet of coax shows.. guess what? the SAME RESULTS.

IT IS possible to get near "perfect world" results with a little work,
and attention to detail.

By the way, I've gotten similar results on two meters with a magnet
mount 5/8 2M Lakeview whip on my old Ford LTLA9000. Had to shorten the
coax because I was sick of the bundle of it in the cab... no change in
SWR over a four MHz bandwidth...

There goes the "magmount theory" as well..

If changing the length of the coax changes the SWR, then there is
something wrong with the antenna.

-SSB

SSB;
I know this was addressed to Tnom, hope you don't mind my 2-cents
worth.

Very few mag mounts have an adequate "RF" ground, at least on 11
meters or lower. There isn't enough capacitive coupling to the
vehicle body. In some cases the length of the coax can matter, not
from a matching point of view, but as a counterpoise for the antenna.
I have seen mag mount installations where you could see the SWR change
just by moving the coax around.

This is true but is totally beside the point of why manufacturers
reccomend 18ft of coax. If you have the problem you mentioned fix the
ground dont play with the coax length. A good test for a CB antenna is to
temporarily splice in 6 ft of coax . If your SWR changes your ground is
probably bad.Poorly made SWR meters will cause this too.

The 18ft rule is also there for "just in case" . As you change the lenght of
the coax the impedance seen by the radio will also change. Assuming the
antenna is at resonance you will see a pure resistance at the radio for only
2 lengths, an odd 1/4wl(16ft) and an even 1/4wl. At all other length unless
the SWR is a perfect 1:1 match the impedances seen at the radio end will be
complex. This is not always a bad thing. However there are some complex
impedances that the radio may not like. It may dislike it so much the finals
fry, the modulation becomes mushy or rx is nearly dead or it may be no
problem at all for any length on your radio. There are a lot of different
radios out there so the manufacturer usually recommends something safe they
figure will work with all radios. 18 ft is a length that will normally allow
enough cable to reach just about anywhere on the vehicle and provide a radio
friendly impedance at the other end of the cable. Does it really need to be
18ft, probably not.. If you find the rare case where it does you can chase
your tail till hell freezes over.

On Fri, 05 Nov 2004 19:09:51 GMT, "Jimmie"
wrote in :

snip
.... Assuming the
antenna is at resonance you will see a pure resistance at the radio for only
2 lengths, an odd 1/4wl(16ft) and an even 1/4wl. At all other length unless
the SWR is a perfect 1:1 match the impedances seen at the radio end will be
complex.


The input impedance to the antenna (and coax) is complex regardless of
resonance simply because the feedpoint is slightly out of phase with
ground (the antenna wouldn't work if it wasn't). Even if the antenna
-was- purely resistive at resonance the impedance would be on the
order of 36.5 ohms (ideal), which does -not- present a 1:1 match to a
50 ohm coax. If you are getting 1:1 SWR then either the antenna has
some sort of matching mechanism or it's a dummy load.

But you are right, using 18 feet is a quick-&-dirty fix for a poor
installation.






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On Fri, 05 Nov 2004 19:09:51 GMT, "Jimmie"
wrote:


"Jimmie" wrote in message
. com...

"Lancer" wrote in message
ews.com...
On Thu, 04 Nov 2004 21:59:02 GMT, sideband wrote:

tnom:

Explain this, then... CB radio in my Peterbilt, with a mirror mounted
Hustler HQ-27 on the driver's side.. 6 feet of coax shows a 1:1 on 19,
1.1:1 on 1, and 1.1:1 on 40... 12 feet of coax shows the same
readings... 18 feet of coax shows.. guess what? the SAME RESULTS.

IT IS possible to get near "perfect world" results with a little work,
and attention to detail.

By the way, I've gotten similar results on two meters with a magnet
mount 5/8 2M Lakeview whip on my old Ford LTLA9000. Had to shorten the
coax because I was sick of the bundle of it in the cab... no change in
SWR over a four MHz bandwidth...

There goes the "magmount theory" as well..

If changing the length of the coax changes the SWR, then there is
something wrong with the antenna.

-SSB

SSB;
I know this was addressed to Tnom, hope you don't mind my 2-cents
worth.

Very few mag mounts have an adequate "RF" ground, at least on 11
meters or lower. There isn't enough capacitive coupling to the
vehicle body. In some cases the length of the coax can matter, not
from a matching point of view, but as a counterpoise for the antenna.
I have seen mag mount installations where you could see the SWR change
just by moving the coax around.

This is true but is totally beside the point of why manufacturers
reccomend 18ft of coax. If you have the problem you mentioned fix the
ground dont play with the coax length. A good test for a CB antenna is to
temporarily splice in 6 ft of coax . If your SWR changes your ground is
probably bad.Poorly made SWR meters will cause this too.

The 18ft rule is also there for "just in case" . As you change the lenght of
the coax the impedance seen by the radio will also change. Assuming the
antenna is at resonance you will see a pure resistance at the radio for only
2 lengths, an odd 1/4wl(16ft) and an even 1/4wl.

Nope, 18 feet is not an electrical 1/2 wave at 27 Mhz. If the antenna
is properly matched and set up, (not just at resonance) any length of
coax should work.

(setup means, good rf grounds etc)

At all other length unless
the SWR is a perfect 1:1 match the impedances seen at the radio end will be
complex.

Like a mag mount with an imperfect coupling to ground?

This is not always a bad thing. However there are some complex
impedances that the radio may not like. It may dislike it so much the finals
fry, the modulation becomes mushy or rx is nearly dead or it may be no
problem at all for any length on your radio. There are a lot of different
radios out there so the manufacturer usually recommends something safe they
figure will work with all radios. 18 ft is a length that will normally allow
enough cable to reach just about anywhere on the vehicle and provide a radio
friendly impedance at the other end of the cable. Does it really need to be
18ft, probably not.. If you find the rare case where it does you can chase
your tail till hell freezes over.