DTR 650 attempts at external antennas

[kb5udf]

Hi all,

There have been various posts about trying to extend the range of the DTR-650 by the use of an external antenna. I have a few observations to share
and a few questions to share with others. I'll start with the question 1:

Question 1: Have you used the new model (DTR600 or DTR700) DTR radios and achieved greater range with an external antenna? If yes, how much more range did you obtain?

Question 2: Have you used the older models (e.g. DTR-650) with an external antenna and achieved greater range? If yes how much more.

Observation 1: I have a tower mounted commercial vertical (cometelco) rated 5db or so, connected to a commercially made run of about 70 feet of LMR400 with N connector, to what appears to be a well made adapter to SMA. This setup greatly extended the range of a 900mhz MTS 2000 (HT) to another HT while I drove about. Radio to radio in my drive which can be described as flat land, urban/suburban mix, but with ALOT of trees compared to many places. HT to HT reception was 1 mile or le ss. HT to tower mounted antenna about 2.5 miles. Again, this is what I have come to expect her on upper UHF frequencies; you get much LESS than line of sight in my area due to foliage attenuation, unless you are above the canopy. As an aside, this same 900mhz antenna hooked up to a 1090mhz filtered dongle, receives ADSB out to 250 miles, so I have reasonable confidence it is working OK.

Observation 2: Hooked a DTR-650 to the same setup, with same coax and adapters, with the tower mounted 900mhz antenna (about 30 feet up), and driving around, I get much less range, then I get HT to HT with built in antennas (the full sized ones that are 7 inches or so). I have tried various adapters and various DTR-650 radios, and none work well on the tower mounted antenna. Some adapters seem to not work at all, given reception dies around the block, some give me about .7 miles of range. This suggests that the antenna jack on the radio is a little picky or different somehow.

Observation 3:One day I took at DTR650, and used the belt clip holster to put it on the tower directly at about 30 feet. Now I got the same roughly 2.5 mile coverage as the MTS2000 achieved with the tower mounted antenna. Ahh, now this is working.

Conclusions/summary: 1. There is something different about the antenna jack, at least I suspect there is, such that not all adapters work. 2. The radios perform poorly with a large, external antenna. 3. Given how they work, its very difficult to test the radios. 4. I wonder if the radios become "deaf" and/or "overloaded" when hooked to a large antenna, in a fashion similar to the HAM grade HT's I used back in the late 1990s. 5. One of these days, I may try this again, as a field operation in some remote area, free from cell towers, and random 900mhz ISM stuff.

Anyone make this work?

Regards

 

[alcahuete]

I spent a great deal of time and effort on this a few years ago with the old DTR650 radios. I used a handful of commercial mobile antennas (mostly Larsen), tiny amount of coax to the mount (actually tried it with 2 ft. at one point), and all failed miserably. It doesn't matter what I tried or what I did.

I consider my location to be near perfect for testing stuff like this. I am in the desert with flat land and relatively few obstructions. I have had my DTR radios working out to almost 30 miles with the stock 6-ish inch 1/2 wave antennas. For the base radio, I bungeed it to one of my towers via tower stand-off mount at approximately 20'. They were working just fine, so I likely could have gone further, but I ran out of road.

With all the mobile antennas I tried, I barely got 3 miles. I could not break that barrier no matter what I did. I literally tried 4 or 5 different antennas, probably a dozen adapters, different mounts, different coax, you name it. They would all fail at basically the same distance, almost to the foot.

So I gave up.

 

[RFI-EMI-GUY]

I spent a great deal of time and effort on this a few years ago with the old DTR650 radios. I used a handful of commercial mobile antennas (mostly Larsen), tiny amount of coax to the mount (actually tried it with 2 ft. at one point), and all failed miserably. It doesn't matter what I tried or what I did.

I consider my location to be near perfect for testing stuff like this. I am in the desert with flat land and relatively few obstructions. I have had my DTR radios working out to almost 30 miles with the stock 6-ish inch 1/2 wave antennas. For the base radio, I bungeed it to one of my towers via tower stand-off mount at approximately 20'. They were working just fine, so I likely could have gone further, but I ran out of road.

With all the mobile antennas I tried, I barely got 3 miles. I could not break that barrier no matter what I did. I literally tried 4 or 5 different antennas, probably a dozen adapters, different mounts, different coax, you name it. They would all fail at basically the same distance, almost to the foot.

So I gave up.

I remember that mystery.

 

[alcahuete]

I remember that mystery.

Yep, never figured the dog gone thing out, so I just gave up.

 

[mmckenna]

Have you tried a wattmeter into a 50Ω load to make sure you are getting a good connection to the antenna jack?

There's other services in this band, are you sure your receiver isn't getting overloaded with the larger antenna?

 

[KevinC]

Have you tried a wattmeter into a 50Ω load to make sure you are getting a good connection to the antenna jack?

There's other services in this band, are you sure your receiver isn't getting overloaded with the larger antenna?

As they are FHSS it will be a little difficult to measure power out (unless they have some conventional test mode).

 

[mmckenna]

As they are FHSS it will be a little difficult to measure power out (unless they have some conventional test mode).

Right, but should be able to see if there is a good connection.

Power output should be stuck at 1 watt, if I recall correctly.

 

[kb5udf]

mmckenna: Right now receiver overload seems like a strong hypothesis, but the wrinkle with that is that it would not seem to explain alachute's poor range as well. Tough thing about testing these little suckers, as you know, includes how they handshake. So if the radio connected to the tower can reach the portable in the field, but the radio connected to the tower can't hear the portable, you get the same result, a "bonk." I don't have a wattmeter to test that high up (in frequency), but I'll see what my friends can turn up. I supposed I could do a test by taking my SDR and a laptop in the field at a distance, and attempting to visualize peak reception from the tower mounted antenna vs. HT mounted antenna; those little peaks are quick, but SDR sharp can see at least some of them. I suspect the SDR should be a useful test of relative signal strength since it won't depend on the base antenna HT hearing it. I'll probably try that next.

FYI the HT antennas I'm using are just standard MOTO JEDI 900mhz half waves, nothing special; they came with a lot of surplus MTS2000 radios I bought.

 

[mmckenna]

I think it would be interesting to hook up a spectrum analyzer to your antenna on the tower and see how much noise there is.

 

[kb5udf]

I realize its not the same as a piece of commercial gear, but I'll post a screenshot of my airspy. It MAY be a reasonable analogy to how a modest radio like a DTR650 sees the world. I can say that since more cell infrastructure went up very near my tower, 900mhz noise floor is up, and 900mhz trunking signals cheap dongles used to decode are now failing to decode, even though signal strength is adequate. So overload remains a strong possibility.

 

[RFI-EMI-GUY]

Theories:
1) Receiver overload from commercial 900 MHz paging
2) RF impedance at antenna not actually 50 ohms. Where have we seen this before? The HT220 radio which required a gadget to match antenna port to 50 ohms for tuning .

 

[kb5udf]

RFI-EMI-GUY:
1. I checked with an airspy on setup (discone, mincircuits amp, 8 port mini circuits splitter). Only 2 900mhz pager signals and while very strong, being over 10mhz away didn’t do a great deal to the noise floor from 902-928. However, it’s possible they might mess with a DTR differently, but the 2 paging signals are not continuous, so in theory if that was the problem, I would be able to get a quick contact with a DTR. I need to hook my airspy up to my 900mhz tower antenna; pending if I get a chance this week.

2. Impedance, interesting and merits consideration. If the regular Jedi series antennas work on the DTR’s well and that’s what they use apparently, and the Jedi’s work well on the tower mounted antenna, is there really an impedance difference In the radios? Or is it possible the antennas aren’t quite 50 ohm and Jedi’s aren’t quite 50 ohm and the DTR’s are even further away from 50ohm than the Jedis. In that case perhaps if the DTR impedance is further away from 50ohm than Jedi’s are (specutively) then a DTR would work worse with tower antenna.

This brings up a point. I recall from studying antenna theory, or at least I think I do that a half wave vertical, such as these HT antennas claim to be, requires a matching network, which for mobile antennas is a substantial base loading coil. Do these HT antennas have such a coil? If not, then do the radios somehow have a matching network which can work with either the 1/2 wave antenna or the different 1/4wave, and furthermore, is this radio mounted matching network somehow unable to cope with a 50hm antenna (at least in the case of a DTR). But if the DTR can’t cope with a 50ohm antenna somehow, then how does it manage the 1/4wave vertical (unless that little stub of an antenna is also off from 50hm.)

In any event, I will do some range testing with a DTR hooked up to the tower antenna, vs stock antenna, and see how much relative signal strength I can visualize in the field with an SDR; then I’ll probably call it a day. The radios are great “site” radios, but the HAM in me can’t resist tinkering.

 

[popnokick]

If there are impedance questions a relatively inexpensive NanoVNA will provide impedance measurements up through 1.5 gHz.

 

[RFI-EMI-GUY]

Not sure what model antenna the Jedi series uses, however the bandwidth requirements will be wider for 900 MHz Part 90.

Years ago I ordered some replacement OEM UHF range 2 (Green) helical antennas for Systems Saber and what I received from Motorola NPD was a different, superseded part number. (K25 received vs K08 ordered, I think) They were actually a bit shorter than the original and got me to making some range checks, first with field strength meter, then with signal generator. The new antennas were markedly inferior at 462 MHz. I contacted Motorola and complained and was told they were a direct replacement. They even put me in touch with the vendor (subcontracted) who told me they simply make and test to Motorola specs. What surprised me was that they were being tuned at 414 MHz per the spec. The new part number had a wider frequency range to the low end. I really think Motorola goofed up somehow and perhaps were trying to please hams with this antenna, or they just had poor QC documentation and were making them with a finer pitch coil. They insisted they were good, my instrumentation disagreed. I have since gone back to the 1/4 wave whip since the helical is NLA.

 

[prcguy]

I’ve seen some radios that sense a short or resistance on the external antenna jack and that causes the radio to switch to the external jack. Could something like this be going on with the DTR radios?

 

[RFI-EMI-GUY]

I’ve seen some radios that sense a short or resistance on the external antenna jack and that causes the radio to switch to the external jack. Could something like this be going on with the DTR radios?

Tried to glean something from the FCC certification of the radio. They mention a "non standard" connector for DUT #2 to perform certain conducted tests. However no pictures or other details. It would help if a schematic exists.

 

[kb5udf]

I don’t know the part numbers off hand, but field testing clearly indicated that a DTR650 with the long, half wave 900MHz antenna from my surplus Jedi radios outperformed/out ranged the shorter 900mhz antennas, that came with the DTR650 radios that came with antennas. I think that small antenna said 800/900 on the side of it.

In any event, given that half wave antennas are usually fairly broad banded, given that we are at 900, I would not expect any problems going down to 902, and that is exactly what I get, which is improved range.

As for DTR600/700 antennas I can’t comment on that I haven’t used them.

Popnokick, I have a nano van, and comprehend how to test an antenna with it, but can I meaningfully test the radio with it?

 

[popnokick]

What I meant was impedance testing of the antennas (not radio). If there's a way to test the radio with the NanoVNA I don't know how that would be done.

 

[RFI-EMI-GUY]

What I meant was impedance testing of the antennas (not radio). If there's a way to test the radio with the NanoVNA I don't know how that would be done.

I wonder that as well.

Even testing the antenna's themselves with VNA will be tricky because they require the frame of the radio and your hand to create a counterpoise. Less so at 900 MHz than VHF, but still an influence. You might get a damaged radio to make a fixture or get a mating connector and install on a small bud box of similar size as the radio.