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Antenna analyzer and 1kw amp

 
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Antenna analyzer and 1kw amp
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OgreVorbis
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Post Antenna analyzer and 1kw amp Reply with quote
Hi

I tested a 1kW amp into an antenna for the first time after testing on a dummy load. When plugged into the dummy load, I have a temperature of 70C on the mosfet and it stays there for a long time and makes exactly 1kW. When I connected the antenna, I had only 900W out instead of 1kW and the mosfet quickly reached 100C, so I turned it off. I used an antenna analyser before I connected the amp and it was only 1.2. Maybe the antenna analyser power is too low to get the right reading? Do I need to adjust the antenna or maybe something else is wrong?

Thanks in advance!
Mon May 23, 2016 8:33 am View user's profile Send private message
pcs
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Post Reply with quote
If you have analyzer, use it and work on reducing swr further down.

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Marko - PCS Electronics
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Mon May 23, 2016 12:30 pm View user's profile Send private message Visit poster's website
OgreVorbis
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It is hard for me to adjust the antenna where it is, so I want to make sure it is the problem first.

I found that an SWR of 1.2 with 1kW should mean that roughly 10W is reflected. If this is true, then why do I get only 900W on the antenna and 1kW on the dummy load?

Is SWR not the same at different power levels? If I have 1.2 SWR with 1mW or whatever the rig expert makes, does that mean I will have the same with 1kW?
Mon May 23, 2016 11:09 pm View user's profile Send private message
pcs
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Post Reply with quote
Normally I would say it is not a problem, but if you are seeing this huge amount of heating compared to using a dummy load than I would suggest further improving SWR for this reason.

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Marko - PCS Electronics
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Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128
Tue May 24, 2016 9:47 am View user's profile Send private message Visit poster's website
OgreVorbis
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Joined: 30 Jan 2014
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So after I put the amplifier back on the dummy load, I noticed some tiny sparks forming on one of the gate tabs. It was strange because it didn't seem to be sparking to anything. It appeared only the surface was sparking. Anyway, after a few more seconds there was a large spark near one of the drain tabs. I turned it off and inspected. It looks like what happened is the guy who mounted the transistor (this is not one of your amps) put it slightly too close to one side of the slot. The copper on the PCB goes all the way to the edge of the slot, so the ground of the transistor was shorting with one of the drain tabs (at least that's what I think happened). I don't know what caused the small sparking on the input, but I'm guessing it was related.

Anyway, I think the transistor is not fully damaged, but I'm going to order another anyway. I might also take this opportunity to improve the thermal transfer so the SWR doesn't effect it as much.

A few questions:

The MRFE6VP61K25H on your site. Is it the one with the mounting holes?

Do you have a US 120V version of the desoldering tool?

What do you think about soldering the transistor to the copper heatspreader instead of thermal paste? Will it be better?
If not, what it the best thermal paste to use?

Thanks!
Tue May 24, 2016 10:52 pm View user's profile Send private message
pcs
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Joined: 18 Jan 2002
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Location: Radio Land

Post Reply with quote
Seems your amp is of poor quality.

The LDMOS can spark when it fails, also it might spark several times I suppose although I did not see this yet. The flash of death can be seen and it is a bad sign

Desoldering tool is 220V, but you can take the tip out and maybe install it in similar cheap 110V soldering iron of suitable power. I can sell you just the tip.

Our LDMOS has mounting holes.

The heatspreader is very difficult to heatup to required temperature and if you do that you will cook the LDMOS as well.
Instead of very quick soldering you end up with very long one. Don't do it.

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Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128
Thu May 26, 2016 4:20 pm View user's profile Send private message Visit poster's website
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