Programming non-amateur 70cm to amateur band... expected power loss?

[needairtime]

I'm having a large suspicion that I'm suffering a fairly large power penalty when using my LMRS radios in the 70cm ham band. Should I?

Both of my LMRS radios appear to have a factory frequency band of 450MHz-480MHz as they use the 13W Mitsubishi M57704H hybrid, though one uses it as a preamp and other as a final. How much transmitted power degradation can I expect to transmit at, say, 442MHz? 444MHz? Could it be half power? Worse?

My Icom mobile rig seems to be transmitting very little power, and seems to be easily overpowered by my Baofeng, at least from what I can tell from actually moving the radio farther away from the repeater. Not sure if it's due to the hybrid passband or perhaps it has some other issues like a blown final. Zero distance testing of my Kenwood has been done, which uses the same hybrid but as the final. Ιt has been in a fixed location since I got it working in the ham bands...

Any ideas, or pretty much people are using amateur-tuned equipment straight from the factory?

 

[RFI-EMI-GUY]

Have you measured the output power and VSWR of this radio? What model radios?

 

[needairtime]

That's one of the problems, I do not have power measurement gear. I do wonder if a germanium or a switching power supply rated schottky rectifier is sufficient to rectify UHF signals to get a peak voltage rating -- and a 50 ohm dummy load -- to calculate power... I haven't tried as I heard that these may not be sufficient to get an accurate reading. I suppose that not having a regular power meter, having a UHF SWR meter is unlikely either ...
This is a Icom IC-U400 and a Kenwood TK-810L. I have not tried driving my TK-810L around to test against known repeater antennas and physical blockages.
So you're saying that these power drops would be unexpected despite not being in the rated pass bands? Both of these radios were designed to a certain band and looking at the schematics, there indeed are differences between each of the versions...

 

[N4KVE]

In my experience of using 450-470 commercial radios in the 440 ham band, there was no loss of TX power below 450 MHz. However it was the receive sensitivity that suffered once I went below 450.

 

[needairtime]

Interesting, but I wonder why there are distinct versions of the hybrids (and none overlap, as well as not covering the ham band). But indeed I've noticed that the Kenwood suffers that exact issue that you describe, though I was able to improve it after aligning it a bit.

 

[RFI-EMI-GUY]

Without actually measuring the power, you cannot say there is a power loss. In my experience, equipment designed for 450-470 MHz usually will operate considerabley lower into the ham band. However, the PLL may become unlocked because the VCO steering line voltage is out of spec, or the deviation may be out of whack. As N4KVE mentioned, the receiver sensitivity will suffer more of a degradation.

 

[needairtime]

Yeah I hit the PLL unlock issue, was able to resolve that. The only reason why it seems like some power drop is that it was outperformed by the Baofeng. I could move the antenna four feet and then hit the repeater it had trouble with (but the Baofeng could hit the repeater just fine from the same locations). The antenna in its original location was measured to be 1.8:1 SWR when I had access to a meter, but no idea what it is when I moved it from the roof of my car to the hood where it worked... Note also that this is my car in my garage.

It works quite a bit different when driven out of course -- but there are still many spots around town where the baofeng works -- in the cabin "gaussian shield" -- versus the roof antenna.

I really wish I could test in-band, just that I don't have a GMRS license (though the radios are both part 90 and 95).

On the other hand I also should fix up my dummy load, it seems to leak RF, enough so that the local repeater can still pick it up...

 

[needairtime]

Hmm... this may be a bad sign... but some background.

I tried using a Germanium diode rectifier to do a peak detector with the dummy load. I tried it with my 2m Kenwood HT and it seems to correlate with what I see on my scope. The old 300MHz has enough bandwidth for 2m.

Hooked it up to my 70cm LMRS Kenwood... got 9 volts. I have the radio set to low power so this is ... a little low, but still within the ball park. Can't view on my scope and get a reasonable number, so I have to trust it, even though I have to still put a bit of skepticism due to the crap circuit I have to do peak detection.

Now the acid test: trying on the Icom... 13 volts! That's way low, should be way higher, but it may be due to the 1N270 not liking UHF frequencies.

 

[RFI-EMI-GUY]

Hmm... this may be a bad sign... but some background.

I tried using a Germanium diode rectifier to do a peak detector with the dummy load. I tried it with my 2m Kenwood HT and it seems to correlate with what I see on my scope. The old 300MHz has enough bandwidth for 2m.

Hooked it up to my 70cm LMRS Kenwood... got 9 volts. I have the radio set to low power so this is ... a little low, but still within the ball park. Can't view on my scope and get a reasonable number, so I have to trust it, even though I have to still put a bit of skepticism due to the crap circuit I have to do peak detection.

Now the acid test: trying on the Icom... 13 volts! That's way low, should be way higher, but it may be due to the 1N270 not liking UHF frequencies.

Hmm 9.0 volts would be about .9 watts depending on the forward voltage drop of the diode. 13 Volts would be 1.8 Watts (13 + 0.4)/1.414 (9.48 VRMS. 9.48^2/50 = 1.8 watts.

Your diode may experiencing breakdown due to the PIV. For example a 1N5711 is good for about 15 watts. It has a Vbr of 70V and I have read you need significant derateing. You might need a couple or 3 in series. How are you physically tapping the load? What kind of load?

 

[needairtime]

Tapping as close as possible to the 50 ohm resistive load with a 50pF capacitor. The 1N270 is supposedly rated for 100V PIV. I suspect the drop of the germanium diode is larger than the 1N5711 Schottky but I would hope that they would be in the ballpark as the voltages should be much higher as a percentage of the diode drop.

I am using a 47K ohm resistor as the output impedance/other end of the divider, which is significantly larger than the impedance of the coupling capacitor at UHF frequencies, and much larger than the 50 ohm dummy. The DMM is 10M ohms which is much larger than the 47K ohm. A 1nF capacitor is used as the low pass filter.

I need to test what this dummy load+peak voltage jig does with my Baofeng or one of my CBs, which have semi-reliable outputs so I can "calibrate" this jig, but I think 13 volts is way too low for a 35W transmitter no matter what the Baofeng says.

---
Uniden HT CB:
@1W 7.7V
@4W 15.0V

So I think the voltage may be reading a bit low... And the impedance of the coupling capacitor is higher at 27MHz compared to 440MHz so I should get a *little* less than reality with CB. Not that much less since the divider resistor is 47K!

 

[RFI-EMI-GUY]

Ok. Why not tap the diode directly to the 50 ohm load without the series 50 of cap? You need a DC return anyhow so the load can serve that purpose.

Sent from my SM-T350 using Tapatalk

 

[needairtime]

Ah...well, hoping to get rid of any DC offset or 60Hz hum that might show up... 50pF is like infinite ohms (well, not really) to DC or 60 cycle hum.

---

OH yeah... at least for 2m it seems to do something very reasonable:
Using the crystals that I do have in my Regency:
@1W: 12.0V ... this voltage is fairly high... maybe it's actually pumping more than 1W
@15W: 41.0V YEAAH this is definitely pumping out watts. I really need to get my homemade PLL working for this thing.

 

[RFI-EMI-GUY]

You might also try turning on some 460 MHz freqs in those LMR radios to see if you get better power than at 440.

Sent from my SM-T350 using Tapatalk

 

[needairtime]

I'm kind of hesitant to do so, I think my dummy load leaks RF because I was able unintelligibly hit the repeater (the local repeater was able to pick up the CTCSS tone but I heard all noise. I quickly hooked up the real antenna again, ID'ed, and apologized. I can't do this legally with GMRS frequencies!). Might have to go over the holes with some conductive tape a few more times, or I have to find out where else it's possibly leaking and ensure it at least does not leak in ham frequencies first.

 

[MTS2000des]

Modern radios made since the 2000s generally don't suffer output loss a few MHz above or below the rated bandsplit. It's the receiver front ends that may or may not have some degradation. As been covered by others, without test equipment, it's pure speculation.

Now old 1980s vintage PLL radios like your U-400 are a different story. Many of these require retuning and/or replacement of frequency sensitive components to get rated specifications outside their factory tuned receiver and transmitter bandsplits. Most modern Kenwood, Icom and Vertex-Standard radios for 450-470 and later 450-520 have RF hardware that is fully capable of operating down to 440MHz without any serious performance loss. Getting the programming software and radio firmware to play nice can be another challenge. Some radios, like Kenwoods, will merely accept the OOB frequencies and warn you. Others require hex editing of programming software and code plugs.

 

[needairtime]

I was a bit alarmed because of the bandsplits that were offered for each of the 80s radios being fairly tight (20-30MHz) and perfect split boundaries with no overlap, hence the concern that there's possibly significant loss if going out of the bands. These aren't ham radios so I thought that they weren't designed to be very variable in frequency.

Comparing the schematics for each of the radios seems to indicate clear differences between them, and that amplifier hybrid chip is one of the differences. While the U-400 uses different hybrids, the final is the same between the versions. There are unfortunately lots of passives distributed in the signal path which are different which alarms me more.

One thing I did notice is that the power consumption of the U400 shoots up when "transmitting." While expected, this seems to indicate the power is going somewhere...where, I don't know, if it a bona fide dummy load is hooked up and I don't see the expected voltage drop and thus dissipation in the load. At least it is getting PLL lock, but we already knew that because I can move somewhere with better LoS with the repeater and it would work. Just that the Baofeng can deal with a few walls in between.

 

[RFI-EMI-GUY]

I'm kind of hesitant to do so, I think my dummy load leaks RF because I was able unintelligibly hit the repeater (the local repeater was able to pick up the CTCSS tone but I heard all noise. I quickly hooked up the real antenna again, ID'ed, and apologized. I can't do this legally with GMRS frequencies!). Might have to go over the holes with some conductive tape a few more times, or I have to find out where else it's possibly leaking and ensure it at least does not leak in ham frequencies first.

A GMRS license is CHEAP! About 2 cents a day for 10 years!

Do your testing on the repeater output, nobody will hear you unless your jumper cables are bad. I assume this is a real resistive load good to 500 MHz and not some MFJ concoction?

I used to do a lot of "hobby" radio stuff using very rudimentary equipment. A Bearcat 200XL as a signal generator and deviation meter. You can generate stable (if not accurate) signals from a scanner if you calculate its LO frequency. If you wire a discriminator tap, you can connect an oscilloscope, DC coupled, to the tap. Then calibrate it by turning on a CW source and setting the channel + 5 KHz, then - 5 KHz to set your top and bottom graticules. More recently I needed a signal in the 23 CM band to test an ICOM IC-970 in 1296 MHz range. I had nothing that went that high. I did have a pyramidal horn antenna and a Motorola UHF Expo radio. The third harmonic of its LO was where I needed to be. I turned the Expo on, put it on the floor and set that huge horn over it. I could readily test functionality of the receiver and outboard preamp.

Over the decades I have graduated from that Bearcat setup, to an old Measurements signal generator , excellent attenuator for tuning receivers, awful frequency stability, to a surplus Motorola test receiver, lab built and bizarre. A Motorola S1327B Service Monitor (real money spent) that worked well for years and I even recouped a good bit of what I spent. Finally a lightly used HP8920B which does it all.

At work, I always had access to the latest and greatest test equipment. The Motorola R2001D was by far the easiest equipment to use. However, keeping one going is impossible. I turned down an R2400 that a guy on Craigslist was offering for $75 simply because I knew that even with its probably minor fault it would be a PITA. I am sure he would have let me have it for $25 but they are that bad.

If you are looking for a good service analyzer I recommend the HP8920B with Option 102 or the HP8924 series with proper options. Thousands were made for the LMR and cellular market, they are obsolete now but still can be repaired as documentation and parts are available. Do spend a bit more and get a good one.

 

[needairtime]

My budget for this hobby is $0 unfortunately. Otherwise I'd have a spectrum analyzer by now. I'm just scrounging and using whatever parts I have on hand, and trying to compensate for any deficiencies.

If only I were actually making money from servicing or selling these things...