Background
During our trip to Mt Umunuhm (W6/CC-052) to test radio performance in a high RF environment, we noticed that some people could hear us better on certain radios. At the time we had no way to determine if this was because of the antenna or the radio. This question bugged us, so we decided to find out.
We
decided the best way to determine how a radio/antenna combination works
was to transmit and use an RF field strength meter to measure the
output. (Watch the video here)
Field Strength Meter
A bit of research showed that such a meter is very simple to build, so parts were acquired. We decided to use the VK3YE schematic, which is very similar or identical to others that we came across.
The values for the capacitors and potentiometer are not critical. We used 47n at the antenna, and a 50k pot. The diodes can be any germanium type. We found some 1N34A's for cheap on eBay to use. A regular digital multimeter (DMM) in voltage mode is connected across the 10n capacitor to get the reading. Adjust the sensitivity pot to get the values into a good range. We built ours on a piece of perf board, and used an alligator clip jumper as an antenna.
 | ||
| Testing the meter at home. |
Initial tests in the shack were good. Standing above the desk with an HT resulted in up to two volts. From across the room, there were still a few dozen millivolts on the meter.
It is important to note that
this meter only gives relative field strength readings. If you don't
measure the position of the potentiometer, readings made with it in
different positions are not comparable. At the antenna range, once we
adjusted it to get an appropriate sensitivity, we did not touch it
again.
Antenna Range
Since
we're hams we don't have a real antenna range, but the park down the
street would work just as well. One Saturday we packed up all the radios
and antennas and walked over. We picked a spot in the shade, and got
set up. KN6REU volunteered to be the transmitter and AA6XA logged. The
radios were on the ground, or used in a sitting position, like we do on
SOTA activations. The field strength meter antenna was clipped onto the
handle of the tripod we were using to film, with everything dangling,
and the DMM on the ground. The tripod was about 7m away from the radios.
Far field is generally considered to be three wavelengths or more, so
we were measuring in the far field.
 | |
| Test setup. |
We started with the same radio/antenna combinations we had on Mt Umunhum. We had some trouble initially, there was nothing being registered on the meter. We're not sure exactly what the problem was, but it was likely the alligator clip not grasping well. After testing those combinations, we tried each antenna with the TH-F6a, our main SOTA HT.
Radios and Antennas
Radios tested:
- Kenwood TH-F6a
- Radio Shack HTX-202
- Yaesu FT-817
- Elecraft KX3
- Baofeng UV-5r
All radios were set to their highest power (5W [3W KX3]). All testing was in the 2m band (146.505MHz).
Antennas tested:
- OEM Kenwood Whip
- OEM Radio Shack Whip
- OEM Yaesu Long Whip
- The 817 is secondhand, we did not receive the small tip with it.
- OEM Baofeng Whip
- This has the SMA-RP connector, and we don't have any adaptors, so this antenna could only be used with the Baofeng.
- Comet HT-224 tri-band whip
- RH-770 extendable dual band antenna
- Arrow 2m Yagi, 3 elements
- Extended Double Zepp (EDZ), homemade
We also made a ~19.5 inch "tiger tail" counterpoise to test with the whip antennas. (This is about a quarter wavelength on 2m)
Results
First, note that the meter only gives relative field strength readings. A voltage of 10 or 100 is meaningless by itself.
The first test was each antenna with the radio used on Mt Umunhum.
Some of these were surprisingly low. The 817 was one of the best transmitters on Mt Um, but here it didn't get out as well as the 202. The zero reading for the KX3 was surprising. We thought the right-angle BNC connector we used initially might be bad, so we tried again with it removed, but this made no improvement. We were not surprised the Baofeng did so poorly here. We tried with the tail here, since we couldn't move the antenna, and it didn't help. It is possible the connector did not make a good connection here since the tiger tail was not quite wide enough to fit around the SMA.
Second,
we tried each of the whip antennas with our standard SOTA HT, the
Kenwood TH-F6a. Readings are in mV, without the tiger tail counterpoise
and with it respectively.
The readings without the tail were not surprising. The RH-770 is much longer than the whips, and we expected it to do better. As expected, the performance with the tail was much improved. The jump in the 817 whip's performance is a bit suspicious. We wonder if one of the two readings is wrong. We were also surprised at how bad the OEM Kenwood whip did. Back in the shack, we put the antennas on the nanoVNA (see below). The SWR of the Kenwood was particularly bad, and we could make it jump as we flexed the antenna. It is likely broken internally.
Finally, we tried the "big" antennas, the Arrow Yagi and EDZ. Readings in mV.
By
far, the Yagi was the best performer when pointed at the meter. We also
tried pointing it at 90 degrees to the meter, and directly away. As
expected, the value dropped a lot, showing it is in fact a directional
antenna. We were surprised how low the reading for the EDZ was. We've
had good luck with it on summits, and expected a higher reading. One
possible issue with the EDZ is that sometimes we hang it from a tree,
other times it is vertical next to the pole. Further testing could show
if one mounting method is better.
Conclusions
We did not find anything earth shattering. As you'd expect, directional antennas do better, and longer antennas do better.
The best way to improve your signal appears to be the addition of a counterpoise to the HT. Even with the short and possibly broken whips, there was an order of magnitude improvement in signal strength.
We
are still stumped as to why the KX3 did so poorly both in this test and
the receiving test. We know the internal 2m module works, it has been
used to make contacts and as an IF rig for microwave bands.
 | |||
| So many radios and antennas (Mt Umunhum) |
ham_bitious video of the testing on YouTube.
------------------------------------------------------------------------------------------------------------------------
Appendix: nanoVNA plots
These
images were captured in the shack, after the antenna range testing, so
no plots for the Arrow Yagi or EDZ. We do not have an adapter for the
funky Baofeng connector, so no plots there either. All the plots are
from 144-148 MHz, the American 2m band. The marker is at 146.52, the US
simplex calling frequency. The yellow plot is return loss LogMag (higher
magnitude values better), green Smith chart (closer to the center
better), blue Phase, and purple SWR (lower values better).
The
whips seem very sensitive to how the nanoVNA is held, how close the
hand is, etc., and the return loss could easily change a few dB. As you
can see, I tried to hold it the same way each time. The Kenwood whip is
definitely broken. Moving the top part around would have massive changes
in return loss, e.g. from -10dB to -0.01dB. The right angle adapter
that we used with the KX3 seems to be fine. The last plot is the Comet
antenna with the adapter.
 |
| Kenwood OEM whip |
 |
| Comet HT-224 |
 |
| Radio Shack OEM Whip |
 |
| Yaesu OEM Long Whip |
 |
| RH-770 Extendable |
 |
| Comet with right angle adapter |
No comments:
Post a Comment