~70W VHF linear... reducing input and output power to be usable for 2m?

[needairtime]

It escapes me who the manufacturer is and the exact wattage, but I acquired a probably broken 2 meter linear amplifier. It even seems to have an FCC approval ID on it, but I suspect it was for commercial VHF 2 way radio use and not amateur.

Anyway it supposedly requires around a minimum of ~ 15W signal or something and dumps out a ~ 70 watt signal. I don't have a radio that gets that 15 watts, how difficult would it be to get input lower, to say, 4W or so, and if it only gives a few dB gain from that, even 25 W output would be good -- if this linear works at all -- which was the main reason for playing with it. And of course this would be in the amateur bands, hopefully it won't require too much retuning, it should be a linear after all.

Would something like this be feasible to hack without it distorting the output out of compliance or frying something, including the transceiver?

 

[ka3jjz]

Depends. Did you first research the make and model to see if it can go down to 2 meters? If it can then half your work is done. If not there's a good deal of work needed to retune that amp - if it can be retuned at all. And getting the input drive level down isn't going to be easy, from what little I know of it. And do you have the equipment you would need to measure the output and make sure it's still in compliance?

Do some research first. Mike

 

[needairtime]

Well it was pretty much a trash unit, got it real cheap - if it doesn't work, it's still something to study. I recall a label on it saying it indeed covers something like 130-something to 170-something MHz so it definitely covers the ham band, but it covers a lot of the VHF range.

Question remains, if I hack it such that it will start transmitting when a 4W signal is present, even if the output won't get to 70W because of limited gain in the one stage it has, will I get at least 15+ watts output... and will it get there cleanly? I was thinking that the gain would remain similar to what it was designed for, but since the input "volume" is too low, the output "volume" would also be low - but still more powerful than the original signal.

(Then the other question, if I add another stage in by soldering in components since the board seems to have empty spots for a preamp...)

 

[KK2DOG]

Give it a shot and see what happens. My amp requires a 50 watt input to see 160 watts out but I usually only put about 15 watts into it and get about 100 out.

 

[W5lz]

At a rough guess (very rough), with 4 w in you'd maybe get 20 - 25 out. If anything.

 

[needairtime]

What do you mean by "if anything"? (BTW, 20-25W would be quite excellent!)

One "if anything" that I figure is the 4W is insufficient to trigger the T/R relay in the amplifier. So that lead me needing to figure out how to reduce the amount of power needed to trigger the relay.

Or is there another "if anything" I'm neglecting here?

 

[prcguy]

If the amp is rated 15 watts in for 70 watts out it has about 6.7dB gain. For 4 watts in 6.7dB gain would be 18.7 watts. The amp is probably saturated at 70 watts out where the gain might be a little less, so maybe it would be closer to 20 watts with 4 watts drive.

 

[W5lz]

needairtime - I have no idea if 4 watts would 'turn on' that amplifier, that's a function of the sensitivity of the detection circuit and that's not standardized at all. And that was the reason for the "if anything". Can there be anymore of those "if anything"s? Sure, but it would require the schematics and would require a lot more effort to figure out than I' going to put into it... sorry.

 

[ka3jjz]

I'd also be a bit concerned about spectrum purity. If it's a wide band amplifier you might need to add something to keep a 4 watt 2 meter signal (which goes to maybe 20 watts output) on 2 meters and nowhere else. A spectrum analyzer would almost be a must if this is the case....Mike

 

[prcguy]

If the amp has RF sensing it would certainly be meant to use with a transceiver and would have a low pass filter. For VHF the filter usually kicks in somewhere above 200MHz and the low pass filter tuning should be fine over the entire VHF commercial band and 2m amateur. The only caveat is will the amp work at peak performance on 2m and can it be tuned there if needed.

I was just tuning some commercial freq amps down to amateur band a few weeks ago and its not a big deal, you just need a wattmeter and amp meter on the power supply and tune for max output while favoring the least amount of current on the output adjustment.

On the question of adding a gain stage, that's not a beginners project. Some commercial amps like TPL or Henry Radio, etc, have modules they add for different power levels where you can have a single module 20 watt in 60 watt out amp, then they will add a driver module to that to make a 5 watt in 60 watt out and so on. If you were lucky enough to find factory made internal amp modules from your amp company with the correct gain and power levels I would say go for it. I've done it but I also have a pile of amps to pick parts from. Without that you would be best off finding a small external amp to drive the larger one.

I'd also be a bit concerned about spectrum purity. If it's a wide band amplifier you might need to add something to keep a 4 watt 2 meter signal (which goes to maybe 20 watts output) on 2 meters and nowhere else. A spectrum analyzer would almost be a must if this is the case....Mike

 

[needairtime]

Yep indeed, a glance at the circuit board it indeed has a low pass filter on the output, has three inductors and four capacitors for filtering. As it has a predefined range (I forgot that 50MHz was also a VHF frequency, but this thing starts at 130ish MHz so it's definitely not wideband) it should be well filtered for this upper part of VHF.

The board has a spot for another RF transistor and pads for capacitors versus a plug in module, so I suspect it had plans for a preamp and the tracks were probably designed to have the right impedance match though agreed it's no walk in the park to recreate that circuitry based on just the tracks.

Hmm... well, I guess we'll have to see if the transistors are any good.

BTW TPL sounds familiar. I need to get my hands on the amp again.

 

[prcguy]

Sometimes amp mfrs go the other way and add an attenuator to a low level input amp to make it higher level input. I traded for a Mirage 2m amp that was around 10w input for 160w output and it had an input attenuator. I bypassed the attenuator and then had the 2w input version which is more useful to me.

Yep indeed, a glance at the circuit board it indeed has a low pass filter on the output, has three inductors and four capacitors for filtering. As it has a predefined range (I forgot that 50MHz was also a VHF frequency, but this thing starts at 130ish MHz so it's definitely not wideband) it should be well filtered for this upper part of VHF.

The board has a spot for another RF transistor and pads for capacitors versus a plug in module, so I suspect it had plans for a preamp and the tracks were probably designed to have the right impedance match though agreed it's no walk in the park to recreate that circuitry based on just the tracks.

Hmm... well, I guess we'll have to see if the transistors are any good.

BTW TPL sounds familiar. I need to get my hands on the amp again.