KAT100 Alive and Healthy!

[Roy Morris-6]

Earlier I had stated my KAT100-1 was displaying random SWR LEDs on 30 meters.  My antenna is a Carolina Windom 80 with approximately 70 feet of RG8X coming off the bottom of the lower balun to the shack.  (There is an additional 22 feet from lower balun to the off-set center conductor.)  A light bulb turned on in my head and told me to try some extra length of coax on the feed line.  I added an extra 4 1/2 feet which the KAT100-1 really liked.  Now it tunes 1:1 SWR on all bands and the KAT100-1 works to perfection (only the leftmost green LED lights up).  The Bird 43 wattmeter between the KPA100 output and KAT100-1 input verifies this.  Sometimes a simple solution is right under your nose.  Roy Morris  W4WFB
No virus found in this outgoing message.
Checked by AVG Free Edition.
Version: 7.1.394 / Virus Database: 268.8.3/360 - Release Date: 6/9/2006

_______________________________________________
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

[Don Wilhelm-3]

Roy,

That goes to prove once again that "my feedline tunes may antenna" due to
the transformer effects of the feedline length.

The major problem with a multi-band antenna is finding a feedline length
that provides an adequate match for the tuner on all bands - but I wonder
just how high the SWR is on the RG-8X and how much signal is getting lost in
that feedline.

73,
Don W3FPR


> -----Original Message-----

> Earlier I had stated my KAT100-1 was displaying random SWR LEDs
> on 30 meters.  My antenna is a Carolina Windom 80 with
> approximately 70 feet of RG8X coming off the bottom of the lower
> balun to the shack.  (There is an additional 22 feet from lower
> balun to the off-set center conductor.)  A light bulb turned on
> in my head and told me to try some extra length of coax on the
> feed line.  I added an extra 4 1/2 feet which the KAT100-1 really
> liked.  Now it tunes 1:1 SWR on all bands and the KAT100-1 works
> to perfection (only the leftmost green LED lights up).  The Bird
> 43 wattmeter between the KPA100 output and KAT100-1 input
> verifies this.  Sometimes a simple solution is right under your
> nose.  Roy Morris  W4WFB

_______________________________________________
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

[Ron D'Eau Claire-2]

Don makes an excellent point. RG8X is supposed to be very low loss cable,
but it's still 50 ohm transmission line and all low-impedance lines have
relatively high losses at high SWRs.

My data shows that RG8X has a loss of 1.0 dB/100 feet at 10 MHz when
properly matched. That's quite good, and Roy's installation with 70 feet of
line would show only 0.7 dB loss on 30 meters *if matched*. The fact that
changing the length of the transmission line changes the impedance at the
rig demonstrates that his line is not matched: it's terminated in something
other than 50 ohms resistive.

The SWR at the rig end is not an accurate indication of the actual SWR on
the line, even if the SWR meter is put at the end of the coax after the
tuner. That's because the SWR at the rig end will always indicate lower than
the actual SWR at the antenna end of the feedline due to the losses in the
feeder. To assess losses accurately, the SWR must be measured at the antenna
end of the feeder. That's usually not so convenient to do.

If that 70 foot length of RG8X is seeing an SWR as low as 3:1 at the
antenna, the loss in the feedline will be nominal, about 1.1 dB. But if the
SWR is, say, 20:1 at the antenna, the losses in that 70 feet of RG8X will be
more than 5 dB! In that case, putting 100 watts into the feedline at the
transmitter would result in about 30 watts reaching the radiator. That's
still enough two "work the world" when the band is open, so on-air reports
are pretty meaningless. Still, it's a huge drop from 100 watts.

Ron AC7AC



-----Original Message-----
Roy,

That goes to prove once again that "my feedline tunes may antenna" due to
the transformer effects of the feedline length.

The major problem with a multi-band antenna is finding a feedline length
that provides an adequate match for the tuner on all bands - but I wonder
just how high the SWR is on the RG-8X and how much signal is getting lost in
that feedline.

73,
Don W3FPR


_______________________________________________
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

[Don Wilhelm-3]

Actually it is not difficult to measure the SWR (or impedance) of an antenna
at its feedpoint.
Remember that an electrical half wave transmission line repeats the
impedance at the far end, so I keep a half wave of RG-8x for many bands
especially for this purpose (the number of bands grows as I add antennas).

Determining the elctriical half wavelength is not difficult with and antenna
analyzer - start with a cable longer than the length needed and just prune
for the length until the reactive part of the imedance goes to zero -
Theroretically, that can be done with the far end of the line either shorted
or open, but my MFJ259 has trouble properly indicating impedances greater
than 650 ohms, so I use the shorted far end and cut the line for R=0 and X=0
or as close to that as I can read.

Connect the half wave line to the antenna feedpoint and the analyzer will
read the actual feedpoint impedance of the antenna.  It is a neat way to
tune single band antennas to match the feedline that you intend to use.

The impedance of the half wavelength line is not critical - use any coax
that you have available, it does not have to be the same as the feedline
that you will eventually use.

73,
Don W3FPR

> -----Original Message-----
>
> The SWR at the rig end is not an accurate indication of the actual SWR on
> the line, even if the SWR meter is put at the end of the coax after the
> tuner. That's because the SWR at the rig end will always indicate
> lower than
> the actual SWR at the antenna end of the feedline due to the losses in the
> feeder. To assess losses accurately, the SWR must be measured at
> the antenna
> end of the feeder. That's usually not so convenient to do.
>

_______________________________________________
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