FW: [AMRadio] bad antenna luck


Geoff/W5OMR ars.w5omr at gmail.com
Tue Jan 9 10:27:47 EST 2007


Jim candela wrote:
> David,
>
>    There are days when I cannot make a resistor work, or a light bulb light.
> Sounds like you are having a bad antenna day. In times like this I usually
> punt and go back to basics. Some ideas:
>
> Use your transmitter, swr meter and a known 50 ohm dummy load. Do you see
> 1:1 SWR? Answer should be YES. If not find out why.
>   

Here's a question, in that regard... I've got a 640w 'dry' dummy load 
comprised of (4) 50 ohm, 160w NI resistors connected in series/parallel. 
I always ass/u/me'd that it was 50 ohms, but I -finally- put in one of 
those self-righteous, swr-protected, solid-state transceivers and 
watched the output meter while I tuned across the spectrum.

The SWR on my load didn't get below 2:1 (and only got down to 1.5:1) nor 
did the power output creep past 10w (up to around 75w) until the 
frequency readout showed around 2.5Mc.  Everywhere else was 3:1 or better.

Why would (4) 50ohm, 160w Non Inductive resistors lashed up in 
series/parallel show such behavior?


David:

If you've got a solid-state rig with built-in SWR protection, you can 
just use the rig, a little power (30~40w on CW) output into a dummy 
load,and see if the load is working.  You should get no variance in SWR 
from your dummy load.  If that works, then connect the antenna (nothing 
else) and tune the thing across the area of your antenna that's supposed 
to be resonant.  Antenna only.  No tuner, no swr bridge - just the coax 
from the inverted vee into the solid state rig.  Tune around and see 
where the power starts to rise and SWR drops down.  Use the internal 
meters.  Don't have anything else connected between the output of the 
radio and the antenna.
When the power rises, and the SWR dips, that's where the antenna is (at 
least close to) resonant.

If you can't find that point, then I would suggest pulling the antenna 
down, and re-working the feed-point with possibly some new coax.

BUT, if that little experiment worked, then you can move on.

So, now you've found where your antenna is resonant at. 

1) is it where you wish to be operating
Yes:  Go to 2. 
No:  Adjust the length of the antenna.  Sun stretches wire length over time.
        Check your coax feeding the inverted vee.
        Check the connections at the feed-point. 
        Make #1 -happen- before you go on.

2) Re-add SWR Bridge.
SWR OK?
Yes: Go to 3
No: replace coax jumper between rig and SWR Bridge.

3) re-add Tuner
SWR Tuneable to 1.1:1?
Yes: Go to 4
No:  Check your coax jumper between SWR Bridge and Tuner (if it's not 
built in)
(IF SWR Bridge is built into the tuner then maybe the tuner itself has 
decided to stick it's legs in the air and cry out "Uncle".  Check the 
tuner against something known good.)

4) Connect T/R relay to solid state rig. Do NOT connect the power for 
the relay coil.  Connect it on the 'receiver' side of the T/R relay.  Do 
put the coax jumper with associated chain (SWR Bridge/tuner/dummy-load) 
on the 'antenna' connector of the T/R Relay.  Run a little power again, 
and make sure you've got power coming out of the T/R relay.  If THAT's 
good, then turn it around, and connect the coil up to the proper 
voltage.  energize the coil, and run your power experiment again.  Still 
getting the proper power reading and SWR readings through the associated 
hardware?

You can see where this is going, Dave.  I'm trying to eliminate -every- 
possibility that could possibly pop up.  Not because I'm trying to be a 
smart-ass, but because *MY* own hard-headedness has taught me, over the 
years, to look at the most simplest of things.  And, it really is -that- 
easy.

Montgomery Scott, Chief Engineer of the Federation Starship "Enterprise" 
one time said of one of them there 'newfangled ships', (and I've got to 
believe that this is one of my all-time favorite movie quotes):

"The more complicated you make the plumbing,
the easier it is to stop it up".  :-)

Back to Basics, Dave.  Start with your solid-state rig into a dummy 
load.  Is the coax jumper and dummy load working?  Then, go from there.


--
73 = Best Regards,
-Geoff/W5OMR
(it was 30F in South Texas this morning!)



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