XPR6580s weak transmit power low in 33cm band

[KD9DJC]

Hey all,

Been playing around with some XPR6580s I purchased. Did a hex edit to make a code plug with channels in the 33cm ham band. I noticed that I was having major issues with transmit power the lower I went in the ham band.

From about 902 to 910 Mhz, the radios transmit (and maybe combination of receive as well) performance is so weak that I can't even break squelch on another handheld when transmitting on high power. Hooked it up to some test equipment, it shows I'm getting 3W output power from the antenna connector, but still very weak.
I tested this also with an RTL-SDR to make sure that it wasn't just the receiver that's being weird. Transmitting lower in the ham band seemed to appear to show up weaker in SDR#, though I can't say I've entirely ruled out the receiver being the issue here.

Higher in the band, the radios seem to perform just fine. I can get about half a mile range of simplex in a city around buildings. Wondering if anyone else has this issue and if there's a way to address it. I assume it's just because the radios were not designed and tuned to be used on the 33cm band, and the higher frequency in that band, the closer you are to a band it was made for. This is obviously quite detrimental to using repeaters because the repeater inputs are all around 902-903. My XPR 4580 also seems to have the same issue.

 

[PACNWDude]

Motorola, like many companies uses components that meet certain specifications. A chip in a 800/900 MHz radio will be at a tighter spec than one used in a UHF radio, which will be better than one used in a VHF radio......I have seen this with XPR6580 and XPR7580's.

That is all this is. 928 MHz is where I have noticed many hex edited XPR6580's doing well, and then capability drops off quickly (possibly the same chips as their 900 MHz variants), closer to their design specifications, yields higher output power.

 

[16b]

I am confused.... you say that you measured 3 watts of output power. The spec sheet for these radios says 2.5 watts on high power, which in my experience is pretty standard for 900MHz commercial radios. So, at least from the standpoint of output power, your radio is working quite well out of band.

If you are concerned about the receive sensitivity, the only really useful thing to do would be to find someone who can measure the sensitivity on a service monitor. Everything else is just guessing.

Edited to add: you understand that commercial 900MHz radios aren't going to receive on the input portion of the band (902-903), right? That is why the commonly used band plan for 33cm places the repeater outputs and simplex operations at the top end of the band (927-928) and the repeater inputs at the bottom.

 

[K9KLC]

How are you testing this wattage output and receive? Have you hooked them up to a service monitor and checked?
The 6580s I have do about 2.5 watts "or close enough" down around 902 and I don't really try to receive down there that's beyond what they were made for. If you're trying to check a repeater, how far are you from it? What is its capabilities?

The 900 band is finicky and can go up and down in a heart beat.

Edited to add: you understand that commercial 900MHz radios aren't going to receive on the input portion of the band (902-903), right? That is why the commonly used band plan for 33cm places the repeater outputs and simplex operations at the top end of the band (927-928) and the repeater inputs at the bottom.

This is exactly correct. It's not made to receive down there, even in the ham bands we use 'for instance" 927.0125 as a repeater output and 902.0125 as the input. We have others on the air and ALL of them receive at 902.xxx and transmit up at 927.xxx. (the actual repeaters).

The XX80 radios do well for the ham bands as long as you using them properly. If they receive at all at 902, it will be pretty horrible or at least much lower than the spec at 927.

 

[N4KVE]

Once these radios get down to 925 MHz in receive, they’re stone deaf. Basically when they TX from 902-903, 927-928, & RX from 927-928, they’re pretty close to spec. Anything outside of that is a roll of the dice.

 

[K9KLC]

Once these radios get down to 925 MHz in receive, they’re stone deaf. Basically when they TX from 902-903, 927-928, & RX from 927-928, they’re pretty close to spec. Anything outside of that is a roll of the dice.

This is exactly the range we use them. We've used 6580's, 7580's, 4580's and 5580's with great success as long as they're in the frequency range you mention. I've got a couple of SL-7590s for DMR and they're almost exactly the same in the ranges as the XPR's they work in also with maybe a little better range due to those only being mainly for 900 MHz.

 

[N4KVE]

This is exactly the range we use them. We've used 6580's, 7580's, 4580's and 5580's with great success as long as they're in the frequency range you mention. I've got a couple of SL-7590s for DMR and they're almost exactly the same in the ranges as the XPR's they work in also with maybe a little better range due to those only being mainly for 900 MHz.

OTOH, my APX4000‘s were tested to TX from 896-941 with no gaps, & RX down to 922, but it falls off quickly below that.