Connecting to your ant with coax

Posted by on Jun 17, 2005; 9:19pm
URL: http://elecraft.85.s1.nabble.com/Connecting-to-your-ant-with-coax-tp379492.html


Connecting to your ant with coax.

True story..... (maybe a little too long. You can jump
way down to the paragraph labeled - The Practical Lesson):

I built the KX1 (20/40m) and the ATU per the manual. Then
built the vertical that Wayne suggest in the ATU manual;
about 30ft vertical (some military surplus mast sections
resting on a coke bottle) and 4 16-ft radials laid out on the
ground.

Hooked the KX1 to the antenna at the base of the antenna and
hit the tune up buttons. It tuned beautifully (both bands)! I was doing the 'Happy Dance'! But I didn't want to sit at the base of
the antenna while I was operating, so I found some old coax
with connectors (about 15ft in length) and set myself up on
card table that far away. Hit the tune up buttons again and
found the SWR was terrible! (I was reading the SWR on the KX1
read out.)

Now I am a used-to-be-engineer, but have forgotten all that I
once knew about antennas and coax - OK, OK, I admit it, I've
become an appliance operator! Anyway, retirement does that
to you.

I puzzled over this for a few seconds and decided that the
coax or connectors were bad. So I substituted in another coax
cable (about the same length). Still the SWR was way out.
(You guys who are still active engineers know what's wrong
already, I'm sure. So I am writing for the old geezers like
me or neophytes to the hobby.)

The Practical Lesson:
Well, this coax length is a 'gotcha' - Here's the deal....

Recall from your studying the license manual that coax can
act as an impedance transformer. Every half-wave length, the
coax transforms the load impedance (that's your antenna) back
to its original value.

Coax has an attribute call 'Velocity Factor' (remember that
one?). This VF is different for each type of coax. What that
means is (practically speaking) that the coax really needs to
be adjusted in length by multiplying that physical wave-length
by the VF.

So what I needed for coax length that was some multiple of
half-wave length. Recall again from your earlier studies that
the simple formula for a half-wave antenna length (ft) is:  
468/freq (freq is in mhz) . So for 40m that length is roughly 66 ft.
NOW.... you have to shorten the coax by the VF to get the final physical length. I was using coax type RG-59 with a VF of .68
(from somebody's chart on the web). So, 66 x .68 = 45ft (approx).

I rummaged around in my junque box and found a 42ft length
of coax with BNC connectors on each end. I made up an
additional short length of coax with a BNC on one end and a
couple of Alligator clips on the other (this to connect the
vertical to the 42ft length of coax).Hooked all that up to
the KX1 and hit the tune buttons again!

SHAZAM!!! (an old Gomer Pile expression) - SWR was back to
near normal! (more 'Happy Dancing'.)

Oh did I forget to mention that this same length will work on
20m too!

(OK, no flames now, but if you engineer types want to expand on
this, go right ahead).

73 de Jerry/k6iii
San Jose, CA
KX1 s/n 1101
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