ATX Walkabout HF Portable antenna ideal for Yaesu FT-817
As supplied by Yaesu, the FT-817
doesn't include an antenna for HF band operation. One of the first
products introduced to address this problem was the Watson ATX Walkabout
antenna. This is sold by UK dealers Waters and Stanton PLC in the UK,
and by MFJ in the US. The antenna is believed to be a Mizuho design,
made under license for Waters and Stanton by Sandpiper Antennas in
Wales. Alternatives have since appeared promising miracles, but the ATX
Walkabout is still the best portable whip antenna for the 817, as well as being the least expensive.
Description
The ATX is a base loaded whip - a conventional, proven design as used
in many mobile antennas. The base loading coil has taps for the
principal amateur bands, 80m through 6m. Resonance is achieved by
selecting the appropriate tap (in effect shorting out the lower portion
of the loading coil) and then fine-tuning by extending or reducing the
telescopic whip. A ground or counterpoise is required to achieve a good
match and get the best radiating efficiency.
A
disadvantage of the ATX design is that the WARC bands don't have their
own dedicated taps. Instead, they are tuned by selecting the next lower
band and reducing the length of the whip. This results in a whip length
of about 40cm on 30m. I wouldn't expect this to be very efficient, so I
was quite surprised recently when carrying out some comparative tests
using the WSPR beacon mode that the ATX got signal reports only 6dB down
on my MFJ magnetic loop - which in other tests on other bands has given
results close to those of a dipole!
The
ATX Walkabout can also be used on 2m. The instruction sheet states that
it makes a 'reasonable' 5/8 wave on 2m. I found that it was more than
reasonable. I observed a gain of 2 S-meter bars gain over a quarter wave
whip, which is pretty impressive!
The
original ATX Walkabout had a BNC plug for mounting on the front-panel
socket of the 817. This version is still available, and the first ATX I
had was of this type. However, the FT-817's front-panel BNC isn't
designed to take the strain, and more than one user has had the center
pin of the plug break off inside, requiring an expensive non-warranty
repair.
Waters
and Stanton later introduced a version with a right-angled PL-259 plug
to fit the rear antenna socket. This is a much better bet. My only
quibble is that when installed the antenna leans a little to one side,
as it uses a standard PL-259 elbow connector rather than a specially
made part, and the 'teeth' aren't oriented to allow the whip to stand up
straight.
The
latest Mark II version of the ATX Walkabout apparently has a 3/8" screw
connector and comes with a set of adapters that allow it to be
converted to an inline BNC, inline PL-259 or right-angled PL-259
fitting.
Counterpoises
To
work effectively and present a good match to the transceiver, the ATX
Walkabout needs to be used with a counterpoise of an appropriate length
for each band. To aid quick setting up I created the following chart
using my first ATX antenna.
Although
the nominally correct length for a counterpoise is a quarter of a
wavelength it must be noted that the optimum counterpoise length is very
dependent on the antenna situation, how close to the ground it is and
what other grounding (such as power supply cables) is present. The
effect of whip length on the resonant frequency is also very sharp.
Therefore you are likely to find that different values work better for
you. This also explains the differences between this chart and the
following one using a tape measure counterpoise, which was made at a
different time and in a different situation.
| Band | Length | Whip Length [1] | Best SWR |
|---|---|---|---|
| 40m | See note [2] | 9 sections + 2" | 1.1:1 |
| 20m | 17'8" (538cm) | 9 sections + 1" | 1.6:1 |
| 17m | 14'7" (444cm) | 4 sections + 1 3/4" | 1.1:1 |
| 15m | 12'3" (373cm) | 8 sections + 2 1/4" | 1.1:1 |
| 12m | 9'8" (295cm) | 6 sections | 1.4:1 |
| 10m | 7'11" (241cm) | 9 sections + 3" | 1.3:1 |
| 6m | 3'11" (120cm) | 7 sections + 3" | 1.3:1 |
| 2m | no c/p | 8 sections | 1.2:1 |
Notes:
- Extended sections are counted from the bottom
- 40m counterpoise comprises 36ft of wire. Approx a third of this starting from the end is coiled (3" dia) adding or removing turns until minimum SWR is achieved.
- Wire is 55/0.1mm extra-flexible PVC jacketed (Maplin code RB09K)
- Counterpoises should be laid out straight as far as possible and away from metal objects. Bends, and proximity with metal objects, will detune them, and it may not be possible to compensate by adjusting the whip.
Measurements
for 30m were not attampted due to my feeling that the ATX would be too
inefficient on this band. Measurements for 80m were not made for much
the same reason, plus the fact that a resonant counterpoise would need
to be longer than the wire available at the time.
A tape measure counterpoise
Some
time later I came across the idea of using a lockable steel tape
measure to make a counterpoise for portable vertical antennas. A tape
measure has the benefit of built-in calibration marks and a retractable
case that avoids the problems of tangled wire. I suspect also that the
flat steel tape provides better coupling to ground, providing more
consistent results than when wire is used. Having used a tape measure
counterpoise I would never return to using wire.
You
can buy suitable tape measures for around £5 ($10) in hardware stores. I
bought a 7.5m tape measure, but as the table shows, a 5m tape measure
would be adequate for bands up to 40m.
I
bought a large alligator clip such as is used to clip a battery charger
leads to car battery terminals. I drilled the end of the steel tape and
bolted the alligator clip to it. The clip allows the tape counterpoise
to be firmly attached to the PL-259 elbow used to mount the ATX to the
rear of the FT-817, or alternatively to the alloy "legs" at the bottom
of the 817 itself.
These are measurements I found:
| Band | Freq | Whip Setting | Calibration | SWR |
|---|---|---|---|---|
| 80m | 3.7 | 9 sections | 660cm | 1.1:1 |
| 40m | 7.05 | 8 1/2 sections | 660cm | 1.1:1 |
| 20m | 14.2 | Fully extended | 480cm | 1.1:1 |
| 17m | 18.1 | 5 sections | 390cm | 1.6:1 |
| 15m | 21.2 | Fully extended | 360cm | 1.1:1 |
| 12m | 24.9 | 6 sections | 330cm | 1.1:1 |
| 10m | 28.5 | Fully extended | 200cm | 1.1:1 |
| 6m | 51.0 | 6 2/3 sections | 120cm | 1.3:1 |
It
is noteworthy that a good match can be obtained on 40m and 80m even
though the length of the counterpoise is a lot less than a quarter wave.
One would not expect such a short antenna to radiate much of a signal
on the lower bands. However, the tuning is quite sharp, suggesting that
the antenna has a reasonably high Q, and should be capable of giving
about as good performance as you could expect from a short base loaded
whip antenna.
SWR measurements
Measurements
were subsequently made using a RigExpert AA-200 antenna analyzer, and a
selection of the results are presented here.
Conclusion
This
is an excellent little antenna that uses tried and tested principles
and gives surprisingly good results. The price - compared to its most
obvious competitors - is very reasonable. There are certainly more
effective portable antennas to be had, but if you really must use a
short vertical for your QRP operations then the ATX Walkabout is
unbeatable.
