Thursday, April 28, 2011

Handheld receiver blocking shootout

Ever since my outing on to Ling Fell yesterday I have been bugged by not knowing for sure whether the problems I experienced with the VX-8GR were really caused by receiver overload or blocking. I like the construction and features of the Yaesu. But a radio that makes you miss some of the contacts you have laboriously sweated up a summit to make is about as much use as a chocolate teapot. I wondered if I could devise a test to give me an idea of the relative strengths of the different 2m radios. I did, and the candidates are lined up in order of merit below, the worst on the left and the best on the right.

The test methodology was crude. I connected each radio to my dual band vertical and tuned in a weakish station: the GB3AS repeater on 145.600MHz, which is normally an S3 signal - fully readable but with some background noise on the audio. I then transmitted a carrier on 144.025MHz using another radio on a helical antenna a few metres from the vertical. I tried two power levels of the interfering signal, 3.5W ("high power") and 0.5W ("low power"), these being the available power levels of the test radio. This 8dB difference in the interfering signal level had different effects on the ability to receive the repeater signal.

I am well aware of the limitations of the test I carried out. In real life SOTA or WOTA use a radio may be subjected to strong in-band signals from activators on other summits but they will not be as strong as the signal from a radio a few metres away from the antenna. A radio is likely to be subjected to strong signals from outside the amateur band such as pagers and other commercial signals, which the bandpass filters in modern radios due to the marketing-driven necessity of providing wideband receive coverage will do nothing to attenuate. Many strong signals may mix together to cause intermodulation effects if not blocking. However, a receiver that can handle a strong in-band interfering signal is likely also to be better at coping with many strong signals being received over a range of frequencies. So I think my test results have some validity.

Beginning with the worst receiver, the results are as follows.
  • VX-8GR. This receiver was the worst affected by blocking. Noticeable desensing of the repeater signal occurred when the in-band carrier was on low power, while a weak noisy "4 by 1" signal was killed completely. The repeater signal cut out completely when the in-band carrier was keyed on high power. Engaging the RX ATT (menu option 1) caused the repeater signal to drop below the squelch threshold so it was not much help though it did reduce the desensing effect on stronger signals.
  • JMT-228. The VX-8 was slightly worse than the Jin Ma Tong JT-228, a £30 Chinese handheld bought on eBay. In fairness, the JT-228 is slightly less sensitive than the Japanese ham radios (judging by the signal to noise ratio on weak signals) which may have helped it a bit. Desensing was noticed when the in-band carrier was on low power, and the repeater signal cut out when it was on high power.
  • TH-D72. The Kenwood TH-D72 may only be third worst (or third best) but in fact it was a whole lot better. No detectable desensing occurred when the in-band carrier was on low power. Some desensing occurred, in the form of a drop in S-meter reading and increased noise on the audio, when the carrier was on high power.
  • GP-300. Excellent performance was given by the Motorola GP-300. No desensing was noticed when the in-band carrier was on low power. There was a very slight but hardly noticeable increase in background noise level when the carrier was keyed on high power.
  • TH-205E. I bought this old boat anchor as a "spares or repair" radio for £6 on eBay for the fun of seeing if I could get it going. With the high capacity battery pack it is about the weight and bulk of an FT-817 and not something I would particularly want to haul up a summit. But no desensing of the repeater signal was observed even when the in-band carrier was keyed on high power, making this the best performing receiver of all.
Out of interest I also carried out the test on my FT-817ND and the Kenwood TM-D710 I use as my 2m base station. The FT-817ND was slightly better than the TH-D72: there was no effect with the low power carrier but the high power one brought a noticeable background hiss on the signal. The TM-D710 performed close to the TH-205E. There was barely any noticeable effect from the high power in-band carrier.

I think the results of these tests, crude though they are, are interesting. The bigger the radio, the more likely it is to have a receiver able to handle adjacent strong signals. Paying lots of money for the latest technology is no guarantee of getting a better receiver. In fact, just the opposite. An ex-commercial handheld or a ham band one from the days when wide band receive coverage was not considered important will work better than the latest marvels.

Were it not that I find the full APRS functionality of the VX-8GR and TH-D72 useful, I'd be tempted to sell both those radios and just use a dumb tracker plugged into the mic socket of one of the others tuned to 144.800. Either I use the VX-8GR for APRS only and carry another radio for making contacts or I must try harder to love the TH-D72. Decisions, decisions. But at least I now have a bit more information to base them on.


Alex Hill said...

I was very sad to see my lovely VX8 at the bottom of the pile in your recent test. Trouble is as you've said it really is a nice radio for its other features. Theres only one thing for it Julian, you're just going to have to make you're own

Oddly enough I was having a google yesterday to see if there were any homebrew plans or kits that could be used as trackers or digipeaters that didn't need a transceiver. In the short time I was looking I didn't find much. Plenty of trackers though.

The TH205 isn't exactly pocket sized and it's probably bigger than my rucksack!

Lynn (D) said...

Byonics was working on an All-In-One based on their TinyTrak4 which I was privileged to try out a prototype unit. Picture a TinyTrak 4 coupled to an internal GPS, a Bluetooth to Serial on the PC port, a transciever (yes, transmit AND receive) paired through a transmit relay to a 10watt power amp all packed into the same Pelican case as their current TT3-based transmit-only AIO. Oh, and powered by 8 AA cells also packed inside the case.

It was a nice unit, but apparently the transceiver/TT4 didn't appreciate being in close proximity to the RF from the amp. It worked really well on lower power in my testing (and pairs well with APRSISCE/32 as well), but when I dialed up the power, apparently things didn't work out so well.

They're going back to see how they can fix it, but I have no idea when it might be available again.

With the TT4 internal, it could be a standalone digipeater, with the GPS it was a self-contained tracker, with the Bluetooth link, you could mount it up a tree and still connect a mobile Internet-capable device via Bluetooth to bring a full-function IGate out to the field.

I really, really wanted it to work!

Lynn (D) - KJ4ERJ - Author of APRSISCE for Windows Mobile and Win32

Unknown said...

Hi Alex. Yes I was very sorry to see the VX-8GR go there too. But I've had my doubts about the receiver for a long time and I had to try to get to the bottom of it.

Lynn, the Byonics unit sounds interesting, but solutions that require several different boxes connected by a multitude of cables and each with their own power supply don't really appeal to me. There are enough things to carry up a summit (or forget to carry up a summit) as it is.

What I really need is something that runs on a device like a Windows mobile phone that is a full APRS client and has a built in AFSK packet modem that can simply be hooked into the speaker and mic ports of any handheld. Hint, hint. :)

Lynn (D) said...

That's actually what I particularly liked about the AIO-4. It was a single box, no wires, with an SMA connector on the top. Hook up an antenna, turn it on, close the case, and you're done.

Connect to the Bluetooth from APRSISCE on my phone, and messaging was fully available via the KISS over BT. Even with nothing connected (or with the phone connected), the AIO-4 was tracking and could also be digipeating.

No wires, just a pelican case connected to an antenna.

It looked like this but with a larger TT4 chip instead of the little TT3. Oh, and the transceiver was larger with the transmit/receive switch through the power amp.

I never heard a price, but it was probably in the same vicinity if not a tad more for the receive capability.

(I hate wires too!)

Ricardo - CT2GQV said...

Modern radios with their wide receive/transmit bandwidth can't have dedicated and real input filters like their bigger/older brothers. Guess that's what cause their "low" performance, it's always a tradeoff...

arguendo said...

Thanks for such an interesting blog Julian, well done. The UV-3R is available to buy with a dual-band aerial for a fiver more, 73, Len, G3XXQ