How to check amp input impedance

[dxing440]

How do you check the input impiedance on the dx500v, to see if its 50ohms. The only way to lower my swr`s, with the amp ON, from over 3, down to 1.3, is to add a 3' jumper on to my 12' of coax, making it 15'.
IT HAS TO BE THE AMP! Ive used 3 diff antennas and got the same reults.
I dont need 15', or even 12', but its the only way to lower the swr with the amp ON! Ive done everything everyone suggested,REDONE ALL THE GROUNDS (plus some), and RESOLDERED ALL Coax ends, and moved the antenna above the cab. The tip is now about 13'high and the antenna is only 5' 5'' tall

 

[fodendaf]

if you know someone with an mfj intellituner they give a capacitance and inductance reading so you would know how much of each to add to the amp. there are impedance calculators on the net to work out the input matching such as Impedance Calculators - Mantaro Product Development Services but one would think that some part in the amp has gone past rated value so unless you have a capacitance or inductance meter that would be of no help. im just guessing but there you go.
if you are worried about the antenna then build a salt water dummy load and try txing into that.

 

[dxing440]

i have a friend who has a mfj259, Would you connect it to the radio connector or antenna connector. Im sure he would know how to do it.

 

[Beetle]

i have a friend who has a mfj259, Would you connect it to the radio connector or antenna connector. Im sure he would know how to do it.

You're checking the INPUT impedance of the amplifier. Which of those connectors is the input?

And be very careful using the MFJ -- they are really sensitive about RF voltages at the SO-239.

 

[dxing440]

realized that was a dumb a$$ question afterwards, I had a Brain Frrrrrrtttt.

 

[binrat]

Whar I've down in the past is put my swr meter between the radio / amp, and start adding cable if needed. I had an amp that I had 18' coax in between to make the radio happy.

edit to add: I use a tentec 1225 cross needle meter.

 

[mackmobile43]

I use a bird meter between the radio and amp with half wave jumpers, take the forward and reflected wattage you find and enter it into this calculator and this is your input swr your radio sees into the amp and with that you can retune the amps input.

That's how you correct the amps impedance.

 

[W5LZ]

To check the input impedance of an amplifier, put an SWR meter between the radio and the amplifier. Or use a directional watt meter and calculate it.
It's safe to say that if changing the feed line length between radio and amplifier makes a difference that it isn't 50 ohms (or very close).
- 'Doc