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Author Topic: SWR balance - RF out to Amplifier in - two meters  (Read 1309 times)
N1KCG
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Posts: 51




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« on: February 06, 2013, 05:23:48 AM »

Supposedly I am advised that I can not take the RF out of an HT, and simply attenuate it, then attach it to the input of a two meter RF amplifier.  That the SWR will cause power to be in the radio, and blow out the internal to the HT finals.

Well I kind of believe that could happen, so am not doing this yet.  Will need to read up on what is happening with the RF power, to see if it could end up reflected into the radio finals and blow out the radio.

Facts are  1 to 5 watts out of HT at 145 Mhz.

Limit of 2 to 4 watts into amplifier for two meters.

Using BNC cables and connectors.    Was going to just put a 3db antennuator inline. 

Asking in the antenna forum - because you might know more about SWR and what happens.
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W5DXP
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« Reply #1 on: February 06, 2013, 05:34:21 AM »

Facts are  1 to 5 watts out of HT at 145 Mhz. Limit of 2 to 4 watts into amplifier for two meters.

If the input impedance to the amp is 50 ohms, why not just adjust the HT for 3 watts output and drive the amp without an attenuator? What amp is it?
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
AA4PB
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« Reply #2 on: February 06, 2013, 05:40:12 AM »

If the attenuator is 50 Ohms, the transmitter input is 50 Ohms, and the radio output is 50 Ohms then the attenuator won't cause any SWR or affect the HT. Don't forget that the attenuator must be rated to handle the power output of the HT and you need a way to bypass it during receive so you don't attenuate the receive signals.
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AD4U
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« Reply #3 on: February 06, 2013, 06:15:45 AM »

Most every (if not EVERY) HT has a a power outupt adjustment in the menu system.  Why not just run the HT on LOW or MID power and drive the amp directly.

Dick  AD4U
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N4CR
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« Reply #4 on: February 06, 2013, 06:41:01 AM »

Quote
That the SWR will cause power to be in the radio, and blow out the internal to the HT finals.

Whoever advised you of that gave you bad advice. A 50 ohm 3dB attenuator will present a 1:1 SWR to your radio if it is driving into a 50 ohm load.

In fact, an attenuator can isolate your radio from a bad load. For example, if you are transmitting (at 50 ohms) into a 2:1 load (either 25 or 100 ohms) and put a 50 ohm 3dB attenuator in line, your rig will see a 1.5:1 SWR.

You can attenuate your signal to drive an amp. It's done all the time.

The real question is, will your HT handle full output for extended periods of time? That will be the stress in the system. Your finals heat up even when presented a purely resistive 50 ohm load. As the others here have advised, it would be better to turn down the output level rather than to burn off power as heat.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
N1KCG
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Posts: 51




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« Reply #5 on: February 06, 2013, 06:46:23 AM »

Yes the HT which is  Baofeng UV-5R has an adjustment for HIGH/LOW power output that switches between 1W our or 5W out, but not sure how accurate that is.

the Amplifier is a Kenwood VB-2530 that goes into PROTECTION led lit up, when I try to transmit with the 1W setting.  

Unfortunately the documentation does NOT say if:

1)  PROTECTION because I do not have a strong enough power supply for the amplifier.
2)  PROTECTION because RF input is too weak, or too noisy, or some other issue
3)  PROTECTION because the RFput out is driving an unbalanced mag mount 2 Meter antenna (that can handle 30 watts).  

thanks for any engagement in discussion on this issue.  as of now its all an experiment, but do not wants to overheat anything.

Might switch to 5W out and just turn volume down, as rumer is the RF output will also reduce.
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N4CR
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« Reply #6 on: February 06, 2013, 06:54:55 AM »

Yes the HT which is  Baofeng UV-5R has an adjustment for HIGH/LOW power output that switches between 1W our or 5W out, but not sure how accurate that is.

the Amplifier is a Kenwood VB-2530 that goes into PROTECTION led lit up, when I try to transmit with the 1W setting.  

Unfortunately the documentation does NOT say if:

1)  PROTECTION because I do not have a strong enough power supply for the amplifier.
2)  PROTECTION because RF input is too weak, or too noisy, or some other issue
3)  PROTECTION because the RFput out is driving an unbalanced mag mount 2 Meter antenna (that can handle 30 watts).

The protection circuit in the amplifier is for it's output, not it's input. Try running it into a 50 ohm dummy load. If it still doesn't work, you may have a bad amplifier. If it does work, you have a load impedance on the antenna that the amplifier can't handle.

Quote
Might switch to 5W out and just turn volume down, as rumer is the RF output will also reduce.

This is not true. FM has the same output power unmodulated as it does modulated. This is not SSB.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
WB6BYU
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Posts: 13243




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« Reply #7 on: February 06, 2013, 07:15:35 AM »

Quote from: N1KCG

Supposedly I am advised that I can not take the RF out of an HT, and simply attenuate it, then attach it to the input of a two meter RF amplifier.  That the SWR will cause power to be in the radio, and blow out the internal to the HT finals.



Then don't listen to any more advice from that person.  You certainly CAN do that.
The only issue is running the HT at full output for a long period of time when you
don't need to.



Quote

Was going to just put a 3db antennuator inline. 



That should be fine.  The only issue is that it will also reduce your received signal
by 3dB.  That might or might not be a problem in your situation, but certainly
you can use the attenuator for testing.

The amp should work with 1W drive if it has a proper load on the output.  Solve
that problem first.
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N1KCG
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Posts: 51




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« Reply #8 on: February 06, 2013, 07:17:58 AM »

This is not true. FM has the same output power unmodulated as it does modulated. This is not SSB.

I thought so too.  But some other comment I read stated that this HT behaves different.

Regardless am ordering an RF 50 ohm small dummy load, since am mostly focused on below 50 watts.  just for test purposes.  always good to have something like that for testing radios, feedlines, antenna, connections, tuners, etc.
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KQ6Q
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« Reply #9 on: February 06, 2013, 08:26:58 PM »

One other consideration that hasn't been mentioned, that bit me back in the 1990's
when I tried to use a 2m amp mobile, driving it with an HT. The high power RF from the antenna
got back into the input because the HT was not shielded, causing the amp to go into oscillation.
Unkeying the HT did not give me back a received signal. Had to power them both off.
If you're going to use an amp with an HT, it's best to do it at home, with an outdoor antenna where
the HT is not in the field of the antenna. For mobile, just get a radio designed for mobile use with the
power level you need.
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N1KCG
Member

Posts: 51




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« Reply #10 on: February 07, 2013, 02:17:52 AM »

One other consideration that hasn't been mentioned, that bit me back in the 1990's
when I tried to use a 2m amp mobile, driving it with an HT. The high power RF from the antenna
got back into the input because the HT was not shielded, causing the amp to go into oscillation.
Unkeying the HT did not give me back a received signal. Had to power them both off.
If you're going to use an amp with an HT, it's best to do it at home, with an outdoor antenna where
the HT is not in the field of the antenna. For mobile, just get a radio designed for mobile use with the
power level you need.

good damn points!!  suppose in a car you have the faraday cage to block rf from the radio. back to the drawing-planning board on this whole path.
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W5DXP
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« Reply #11 on: February 07, 2013, 05:06:04 AM »

... causing the amp to go into oscillation.

I had the same problem with a Heathkit 2m amp and during the time I didn't realize it was oscillating and drifting, I knocked out all TV reception in my neighborhood. I also blew the protection circuitry diodes in my IC-22S.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
N6AJR
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Posts: 9908




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« Reply #12 on: February 09, 2013, 04:53:04 PM »

I'v done it for years.  low powewr out on the HT to input on an amp, output the amp to an antenna. done deal , no sweat.
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N1KCG
Member

Posts: 51




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« Reply #13 on: February 20, 2013, 09:55:59 AM »

I'v done it for years.  low powewr out on the HT to input on an amp, output the amp to an antenna. done deal , no sweat.

what amplifier and what antenna are you using?  are they physically near each other?
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