Three things to consider:
1. Resonance is not at all equivalent to radiation efficiency. Yes, a
high SWR will give you more loss in the coax and tuner, but you can
mitigate that by using better coax. RG-316 is fairly lossy.
2. The computer modeling I've done for the AX1 and similar antennas
says that the radials do most of the actual radiating. The whip and the
coil are virtually a lossy counterpoise for the radial(s). Somebody
here a couple of years ago verified that statement by measuring signal
strength as a function of the radial setup, and his numbers fairly
closely match the modeling I've done.
3. The AX1 does exactly what it is supposed to do, but it is pretty
much the very low end of the antenna spectrum for efficiency. In my
opinion, a better solution for POTA or SOTA is this combination:
a.
https://www.dxengineering.com/search/product-line/chameleon-antenna-cha-ss17-stainless-steel-telescopic-whips/part-type/hf-portable-stealth-antennas?fr=part-type
b. a sloping wire extender for the whip if you want to work 40m
c. https://tinyurl.com/3mhrx9nc (mirror mount for whip antenna on
Amazon ... rotate the clamp 90 degrees and fasten to 4 foot pole for a base)
d. four #20 wire radials connected to the mirror mount base and
staked at the far end to also act as guys for the 4 foot pole. Choose
the length to suit your purpose.
If you fabricate a way to quickly connect the radials to the mirror
mount you can literally set this up single handedly in about ten
minutes. If you make the 4 foot pole in three sections the entire thing
packs up just over two feet long and weighs about 3.5 pounds including
four steel stakes. I'm separately sending you a couple of pictures to
show what it looks like. Total cost is roughly $100 even if you buy
everything new.
The AX1 will set up more quickly and you can use it pedestrian mobile,
but the SS17 will get out better. Everything is a compromise of one
sort or other.
Dave AB7E
On 9/25/2024 3:17 PM, James Bennett via Elecraft wrote:
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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