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Lower SWR With Power
- Thread starter Chicago Savage
- Start date
With no external meter in line, I was observing how the radio's internal meter always reads a much better SWR with the amp turned on.
With no external meter in line, I was observing how the radio's internal meter always shows a much better SWR with the amp turned on.
Copy that. I've yet to hear someone say that they are accurate. I don't know why they wouldn't be. He'll, if anything should be able to show reflected power accurately, I would certainly think a circuit inside the damn radio would be the optimum place to take that reading or measurment.
I bought a Yaesu FT-890 it was way off frequency as that particular vintage would do.
There was a bad capacitor in the local reference oscillator. The SWR meter was way out too. After spending a lot of time doing the alignment I was sick of looking at the internals of this radio and almost did not try to fix the SWR meter.
I did take the a time after taking a break. The procedure required three resistors to set the meter with 50 ohms being the center point of the calibration and 75 ohms being used to set the 2:1 range. It works perfectly and I test it every now and then to be sure.
I love it! Makes the State Troopers real twitchy when you break their radar.
What I am saying is that if the antenna is in tune a 4 watts it will still be in tune at four hundred watts. Harmonics from a dirty radio mess up good reading.
Harmonics and oscillation certainly can. I just dont think those problems should be ignored. Bad connections or other antenna issues can cause similar symptoms that change with power levels.
I have a friend that refuses to solder the shield on a pl259 or weatherproof it. He just rolls the braid back and screws it on.
Every year he ends up with high swr when barefoot. When he turns the amp on the voltage is high enough to overcome the high resistance (corrosion) between the shield and pl259 and the swr comes down. He's also been know to have soft tubes but that's another story.
I didn't say all of that should be ignored. What I am saying is that the SWR should be set with a clean radio or a antenna analyzer.
IMHO, and in your case, it is. You already proved that it reads the same as your external SWR meter.
So Shadetree Mechanic and tba02 answered your question as to why you see a different SWR with the amp on or off. The input impedance of the amp changes when it is on or off. If you look inside your amp, you will probably notice wires (not coax) going to the SO-239 connectors, the wires certainly aren't 50 ohms impedance. They either go directly to the relay or to the circuit board where they attach to the relay.
In the off mode, your signal goes form the radio to the amp relay, flips over to the other side of the relay, and right back out of the amp and on towards the antenna. None of the path way in the amp is designed to provide 50 ohms impedance. The relay is designed to switch electricity (AC and/or DC) not RF. They make relays to switch RF but they are considerably more expensive and not usually found in a CB amp.
https://www.mouser.com/Electromechanical/Relays/High-Frequency-RF-Relays/_/N-5g33
When the amp is on, the signal goes from your radio, into the amp, into the relay, but from there takes a different path. It goes to a matching network and ultimately a splitter. This matching network has an impedance that is much closer to the output impedance of your radio, hence the lower SWR reading when the amp is on.
For an initial tune, your radio's SWR meter or an analyzer will work fine, but you should always do your final tune with your station setup the way you are going to run the majority of the time. If you are predominately going to run barefoot, connect the antenna to the radio and tune for lowest SWR. This configuration will give you the strongest TX & RX (coax and insertion loss goes both ways).
If you are going to be using your amp most of the time, then a couple of things. I would suggest using a low pass filter after the amp to keep the family and neighbors happy and a Common Mode Choke at the antenna feedpoint to keep you happy. The CMC will keep any RF coming from your unbalanced antenna from traveling back down the coax shield and, more importantly, any noise that has accumulated on your coax from getting into your antenna and conversely into your radios's receive. Depending on the series resistance of your CMC, you can lower your noise by 25~30 dB. Which means it will be much easier to dig out those weak signals. I too would run a SWR meter as the last thing in line if you are going to run an amp. This way you can keep an eye on things. Just tune the antenna for the lowest SWR on your external meter.
Just my opinion, YMMV.
Good Luck!
Nice post! Gracias. Clearly understood. BTW, it is a mobile setup.
2019 President McKinley radio, Midland HF747 amp, LMR400UF coax, and a 9' whip.
I've been considering a CMC as well.
Top of the evening, from Denver!