Ramsey FM25B amplification

[crusty]

Hello, i'm new the board, but I've recently bought a ramsey 25B FM transmitter kit. While i'm waiting for delivery i thought i'd collaborate a jpole omni anttena, and also happen to think about a couple of old 1 way 75ohm f-type cable 20 db cable amplifiers that i had laying around. I was wondering if anyone has ever to use those as a RF amplifier, or if it's even possible, i know it will support the bandwidth, being wide band amp, but just wondering if it would be liable for use, of if the volt/impedances would give problems, or even worse fry the final amp out of a brand new FM25B. Any ideas, or links to this type of idea?

 

[Nigel Goodwin]

You can't use a line amplifier on the output of a transmitter, you require a PA amplifer - doing it is likely to fry both items.

 

[crusty]

Well I did get good news from someone over on the ramsey forums, I figured it wouldn't be much more than a small signal amp matched for 75ohm Z, it also has fm labeled on the amp for such frequency use, along with adjustable gain control and a fm trap circuit. when the kit gets here i'll do some volt/amp test between them and see if it really does make any change of output power.
This is part of the reply I got from one of the members from ramsey's board.

The cable amp is a small signal amplfier, not intended to drive a 50 ohm load at any great power level. With that said, The cable amp works good for 1-5 MWs dirve, that is if you want to also amplifiy harmonics up to 1000 MHZ. The radio shack 20 db amp works best when using a couple of MW in as you can get about 20 MWs out of it. If you modify the power supply by putting in a full wave bridge, (taking out the one diode thats in there now,) you can get at least 3.5 volts RF out across 50 ohms with 100 MHZ drive at 5 MW in. This translates into about 250MW on the output, If you replace Two peaking coils in the amp with two ferrite inductors you can hit 300 MW into 50 ohms. Is it worth it, well I guess thats up to the end user, you will have a lot of harmonic amplification, so you need to filter well. If you use the amp with just a short antenna for around the house, it will work good for the application

Looks like i'll have to test out the default power output along with adjusting my balun coil, and antenna placement/tweaking to see what kind of range i get, if not i'm considering using this cable amp after some testing, if not i have a old 2 tube sonar brand linear amp i used to use for CB broadcast that would suit the situation, but it throws out like 200W that's just asking for trouble from the FCC. I do appreciate the input though, thought someone might have tried anything familiar to this!

 

[crusty]

After running some tests, it does work, and rather well too! So for all of you who are using a F style 50-ohm antenna output I would suggest using one with variable out and also if you got time, take out the circuit and see what kinda setup the amp has. The radioshack one that i have seems to use a push-pull configuration, it also like i said has a basic LC unadjustable tank for FM filtration, but has a switch to use it or not. I've been using my old realistic RF/SWR meter to measure the RF power output, and it's amazing effective! I can go up to around 1.20 Watts out with the gain all the way up, and it doesn't clip or affect the SNR hardly at all. But what I like a lot is being able to adjust the RF output power from the lil gain knob on the amp, and not having to use the itty bitty vari-resistor on the FM25B board to control RF power. Out of interests I also checked the temperature on the pre-amp and final RF amp in my transmitter with the amplifier hooked up, it seems to take some of the stress off these transistors and they run pretty cool now. Over all i'm pretty impressed for a cheap radiohack VHF/UHF/FM amplifier, if you need a cheap amplifier for these types of needs i would suggest going with one.

 

[mneary]

Just for grins, have you checked your harmonic frequencies for signals?

 

[crusty]

Just for grins, have you checked your harmonic frequencies for signals?

yep, friend has an mini oscope i borrowed the other day, the signal looks pretty clean, not powerful enough to matter anyways, like i said only 1 watt, and my swr isn't THAT great, so i am getting some loss, also constantly running it through the swr/power meter. I've also wrapped the ramsey with tin foil, and and power feed line, all to an outside ground pole connection to cut back on stray amplifications, the radioshack amp is already in a metal casing, i just grounded that, also the circuit uses the casing as a few grounding leads, including the transistor biasing, which seems to be a pretty good thing. I've also been fiddling with the tank circuit thats built in there to originally block FM freqs(which it really didn't work anyways) and replaced the cap to try and block some freqs above 108mhz so the air guys won't be on my nutz.

 

[Nigel Goodwin]

So that would be a NO then!

 

[crusty]

dunno if this has been posted, but it's hellah helpful.
https://www.ntia.doc.gov/osmhome/allochrt.pdf

 

[crusty]

So that would be a NO then!

guess i'll know if they come knockin.