From: billn@hpcvaac.cv.hp.com (bill nelson)
Newsgroups: rec.radio.cb
Subject: Re: setting up a radio...
Date: 12 Oct 92 23:42:07 GMT
Distribution: usa
Organization: Hewlett-Packard Company, Corvallis, Oregon USA

darrellm@pogo.wv.tek.com (Darrell McGinnis) writes:
: 
: 	I want to fire up my CB again. I'll be running a
: 	homebrew vertical (coxail) dipole fed with ~90 ft
: 	of mini-8 type of coax.

Your first mistake. Get good RG-8, or better, 52 ohm cable. Do not use
the junk from Rat Shack. 

Make the coax as short as possible. Losses mount up fast.

: 	I'll be matching the antenna with a (gamma?)-matching length
: 	of other coax.

No, it sound like you are just using an impedance matching section. A gamma
match uses a capacitor/inductor combination for matching.

: 	How can I minimize phone/tv RFI problems.

Use a low-pass filter to minimize FM RFI. You could also install a high
pass filter (one with a cutoff at about 20-25 Mhz) to minimize AM RFI. I
have never had experience with telephone interference, so cannot give any
advice there.

Of course, if anyone happens to live close by, you are likely to be blamed
for causing RFI - even though it is their poorly designed and assembled 
receiving equipment that is at fault.  However, the FCC requires that you
work together to solve the problem. 

: 	I can buy a Lowpass filter BOOO! HISSS! or what?

Nothing wrong with a good low pass filter. It does not cut your output
(or input) signal significantly.

: 	I've seen ARRL discuss coiling 6-8 turns of coax into a loop,
: 	does this work as  low-pass ? Whats apropriate to 27MHz ?

I suspect that it would work as a lowpass filter. Hams generally are
more concerned with TVI than anything else. 

: 	Also, my radio is an older royce with built-in swr, How can I
: 	calibrate this meter before I use it ?

Forget about calibrating the S-meter portion. The readings are just the
fanciful imaginings of the designers.  The SWR portion should be ok as
is - most of them, that I have seen, appear to be fairly accurate. The
best thing to do would be to use an external SWR meter, at the antenna
terminals while you are tuning the antenna. After you are done, just use
the SWR meter in the radio to tell if something radically changes.

: 	I tried a 50ohm resistor (5watt) but couldn't get the meter
: 	to adjust to 1:1.

If it was wire wound, I am not surprised. It would be 50 ohms resistive
but something completely different when you look at the impedance.

: 	Do I need a special RF dummy-load  to get a reasonable reading ?

You might be able to buy one at Rat Shack that might work ok. You could
also see if someone has a good quality dummy load that you could borrow.

: 	If I can't get a good SWR reading should I go for a PI tuning
: 	network between the radio & the feed line ? & adjust to the 
: 	minimum achieveable ?

Using the SWR meter in the radio for tuning is a mistake. It is not going
to be accurate. You need to use a meter at the antenna terminals. That is
the only way that you will be able to properly match the antenna to the
coax.

Nothing you can do, at the radio end, will change the antenna/coax match.
That must be done at the antenna.

: 	How can I verify the impedance of the antenna system ? 

The easiest way is by using a SWR meter at the antenna. If you get a good
match, then the impedance is correct. Note - depending on the rest of the
system, you may or may not be able to get to a 1.1:1 SWR. Any small defect
in the cable, connectors or final tuning is going to affect the antenna
match. So is the tuning (for frequency) of the antenna. The best you can
do is tune for a minimum indication. An SWR of 1.3:1 is negligible. You
are unlikely to even notice an SWR of 1.5:1.  I would be concerned if it
got much higher - unless your tranceiver has protective circuitry.

Bill