Radio Power Testing - Page 1
Forum section: electronics / rf-microwave

author: Randy222
author_rank: Frequent Contributor
timestamp: September 12, 2024, 03:22:15 pm
content: This is probably gray area comment, but.......I am seeing many vids of people testing radio Tx power using various low cost feedthrough Rf power device. This is not really a good way.I say "Tx power" with caveat. Final radio amp stage is gonna eat power regardless of VSWR and amp efficiency. It's not clear to me how many of the cheap feedthrough power meters display "power" accounting for VSWR (fwd power minus reflected power, etc).Power the radio "remotely" with a quality bench power supply. Document the idle (Rx) and key-up (Tx) amps & volts of the power supply. In general, the actual Tx power = Tx(keyed) - RxThis method also lets you see how your tested antenna system VSWR's impacts the final amp stage of radio Tx.Less easy to do, but it's way more accurate regardless of what freq you use or what the VSWR is.This method helps to validate the feedthrough devices. I will test my MFJ-842.I guess there's a big unknown of radio amp efficiency, so it could eat 50w but only Tx 30w.Your thoughts?

author: joeqsmith
author_rank: Super Contributor
timestamp: September 12, 2024, 03:36:38 pm
content: Quote from: Randy222 on September 12, 2024, 03:22:15 pmFeel free to read through the following thread:https://www.eevblog.com/forum/rf-microwave/cb-and-ham-radio-techs-love-their-bird-wattmeters/If you want to see my home made in-line power meter without trying to navigate that large thread:https://www.eevblog.com/forum/rf-microwave/cb-and-ham-radio-techs-love-their-bird-wattmeters/msg4338097/#msg4338097If the video is too long, here is an early version of it:https://www.eevblog.com/forum/rf-microwave/cb-and-ham-radio-techs-love-their-bird-wattmeters/msg4319371/#msg4319371

author: Randy222
author_rank: Frequent Contributor
timestamp: September 12, 2024, 03:49:38 pm
content: Yeah, I think perhaps supply power reading does not necessarily equate to Tx power of the amp, but at least we can grab both readings as a way to somewhat determine amp efficiency.

author: rfdave#gmail.com
author_rank: Contributor
timestamp: September 13, 2024, 01:10:25 am
content: The bench power supply will tell you nothing useful. The RF power is what you're interested in, not the DC power.

author: CaptDon
author_rank: Super Contributor
timestamp: September 13, 2024, 01:05:41 pm
content: There really is one good answer and you MUST know the VSWR. There are many spreadsheets online for direct reference and there is the mathamatical formula. You don't simply subtract reflected from forward, there is an algebraic relationship. For VSWR under 1.1:1 or even 1.2:1 there is so little difference or degradation that you may as well consider the forward power as an acceptable indication of 'transmitter power'. And remember that forward power as well as return power is lost in the cable. Chances are with a long cable run you may see 10 watts reflected at the transmitter while seeing 20 watts reflected at the antenna base. So for a VSWR of 1.1:1 into a good dummy load with a sensible length cable run (6 feet total for HF/VHF the indicated power IS the transmit power. Your suggestion of looking at the power supply voltage and current between transmit and receive is meaningless as it may include relay current, pre-driver and driver stage current and current lost to bias circuits. Why are you overthinking this? If some idiot bragger says his Childrens Band amplifier is 'putting out 10KW' but do to VSWR it is effectively only 9800 watts does anyone really care?

author: Randy222
author_rank: Frequent Contributor
timestamp: September 13, 2024, 04:21:43 pm
content: I was suggesting more of a radio Tx test and not a full system test. What I mean by that is, when possible, dummy load direct on Tx port, or antenna connected directly to Tx port.I agree power supply is not a place to look to know what the actual radio Tx is, but it can at least tell us the efficiency of the radio.The context I was trying to convey was, just radio Tx, so perhaps best to do that with a quality dummy load connected directly to radio Tx port.In other words, all the radio spec sheets tout a power number, which must be obtained with a dummy load close to the port, no regard to any good or bad lines and antenna you may attach.Also interesting to see what supply power is when you key up on a dummy load, then do it again with the line and antenna connected.