SWR Question

[Bob & Debbie Fish]

Hi Guys,
I put up  Bobtail Curtain antenna for 20 meters for the CQ DX contest
this afternoon. It is fed at the center vertical element. Here is the
problem. It requires a matching network (tank circuit) at the feed
point. So I took my MFJ-269 antenna analyzer out to the feedpoint and
tuned the antenna. I got it to 1.1 to 1. I then went inside and found
that I have a 3.9 to 1 swr measured both with the analyzer and the
internal wattmeter on my K2. I have never measured feed point impedance
before I always just tuned antennas by measuring SWR at the transmitter.
Now, I feel like the guy in the old Chinese proverb with two
wristwatches and never knowing what time it is!
I added some ferrite beads at the feedpoint to ensure that it isn't
common mode currents on the outside of the coax sheild causing this. I
always thought that SWR doesn't change along a transmission line. What
is causing this I wonder, and should I just tune the antenna for lowest
SWR at the transmitter regard less of feedpoint SWR?


Thanks in advance,

Bob  K6GGO
_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Vic K2VCO]

Bob Fish wrote:

> I added some ferrite beads at the feedpoint to ensure that it isn't
> common mode currents on the outside of the coax sheild causing this. I
> always thought that SWR doesn't change along a transmission line. What
> is causing this I wonder, and should I just tune the antenna for lowest
> SWR at the transmitter regard less of feedpoint SWR?

I presume you're using a link-coupled tank circuit? Or did you just tap
it for the coax?

The bobtail curtain has a very high impedance at the feedpoint, as you
know. It's like an endfed zepp in that regard.  So it doesn't need a
low-impedance ground connection, but it does need some kind of ground
return. My guess is that you, with the MFJ tuner in your hand, were it,
especially if you were not using a link-coupled tuner.  So when you
walked away, everything changed!

Possibly a 1/4 wave radial connected to the bottom of the tuner would
tame it a bit.

A link-coupled tuner will be much better at isolating the coax than a
tapped tank.

This is my experience with other antennas fed this way -- I never tried
a BC and maybe I'm all wet!
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com