All super-short antennas will exhibit this sort of behavior ... extreme
sensitivity to installation and environment. In such cases, practically
everything conductive that is part of the installation or nearby becomes
part of the antenna in various ways. However ... if you can get RF
current to flow, it will radiate. Your ATU seems to be able to present
a 50+j0 - ish load to your TX so it likely will work as good as
super-short antennas normally do. BTW: many carbon fiber poles are
conductive at RF.
73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County
James Bennett via Elecraft <mailto:elecraft@mailman.qth.net>
Wednesday, September 25, 2024 3:17 PM
Several months ago I started building up a POTA station, including a
KX3 (with internal tuner), the AX1 Dual Band Antenna, the AXE1 40
Meter Extender, and the AXT1 Tripod Adapter. This afternoon I finally
got the time to see how this antenna plays. I was a bit disappointed.
Here is the setup:
KX3 (with internal tuner), 25 feet of RG-316 with ferrites on the
radio end, five foot tall carbon fiber tripod sitting in the middle of
my back yard; no other resonant antennas on the property.
The KX3 / ATU WAS able to find a match to the antenna on all the
bands. At first I thought that was great. But being a bit curious, I
disconnected the coax and hooked up my Rig Expert AA-55 Zoom. Ughhh.
On 40 meters, with the long radial attached and suspended 4-5 feet
above the yard, the antennas was resonant at 7.574 MHz - that’s a tad
more then the +- 200 KHz stated in the specs. And quite ugly for where
I wanted to operate: 7.052 KHz. So, I added the shorter radial, also
up above the ground, going a different direction. NO CHANGE to the
resonant frequency. Yes, I did have the AXE1 installed, and yes I had
the switch set to the “20m” position.
OK, let’s give 30 meters a try. I set the switch to “17m”, and the Rig
Expert to 10.116 +- 5 MHz. Yikes - the thing was resonant at 8.070 MHz
- very long way from 10.116. Adding the short radial changed the SWR
at that frequency from 1.46:1 down to 1.33:1 but no change to resonant
frequency.
20 meters didn’t fare much better. I set the switch to “20m”, removed
the AXE1, and connected only the short radial. Minimum SWR of 1.34:1
was seen at 15.330 MHz. At my desired operating frequency of 14.052,
the SWR was 6.9:1. Not a very good SWR but one that the ATU can easily
handle.
17 meters was just as bad. With the short radial it was resonant at
19.780 with an SWR of 1.15:1; at 18.116 it showed 6.83:1 SWR. Again,
something the ATU can deal with, but…..
While I understand that this is a compromise antenna and it isn’t
going to be spot on on every band, I’m a bit frustrated in what I’m
seeing here. With five watts output into those kind of SWRs, I’m sure
I’m loosing quite a bit of my power in heating the coax and the
modules in the ATU. I hesitate to go out on a maiden POTA activation
with an antenna showing close to 7:1 SWR at my operating frequencies,
even though the ATU “corrects” that.
Any suggestions here? Is this “normal” for the AX1 and AXE1?
Jim Bennett / K7TXA
Eagle, ID
SKCC #10447T
B.U.G. #301
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