SWR Question

I have three transmitting antennas. This question pertains to the one I use on 30 meters. It's a shortened, loaded dipole: two monopole whips with a center connector sitting about 37 feet above ground. My operating set-up is a QRP transceiver (ELAD FDM-DUO) outputting to an amplifier. I usually operate on 30m at about 150 watts (CW). When I take an SWR reading with my antenna analyzer connected directly to the feedline, I get a reading of about 2.0. The cross-needle SWR meter between the amp and the feedline gives me a reading of about 1.5. The digital SWR meter internal to the transceiver reads 1.15.

Why the discrepancies, and what if anything do they signify?

(I also get variant readings with the full half-wave antennas I use for 40, 20, 17, 15, 12 and 10 meters (20 to 10 is a five-band hexbeam), but the discrepancies are widest with the loaded 30m antenna.)

Thanks in advance.
Mark
VE3BXG

 

Consider that the accuracy of a top-of-the-line Bird wattmeter is plus or minus 5% of full scale and you may have your answer.

 

If there's a bit of standing wave the difference in a couple of feet of jumper cables may be noticed.

 

^^^ Good answers above...

But also, how do the three meters read into say a 50 ohm dummy load and then perhaps into 2, 50 ohm dummy loads in parallel (2:1 SWR)?

Sure, positions along the feed line can impact SWR readings, power levels can impact SWR readings (but often for the worse if something is breaking down at higher power) but three meters may not read the same into the same known loads.

 

The SWR meter in the rig is reading the SWR at the input to your amplifier, so that has nothing to do with your antenna.

The discrepancy of 2.0 from the analyzer vs. 1.5 using the SWR bridge at the same point is pretty insignificant but could be either accuracy variations or the influence of the "ground" connection using the analyzer vs. your "rig." The rig's ground system includes the amplifier's chassis, the patch cable from amp to ELAD, and the ELAD itself, plus any other wiring you have; if the analyzer isn't connected to all that stuff, it's minus all that influence.

But either way: 2.0 or 1.5 is considered a pretty good match, and normally to deal with this I'd have a beer.

 

Thanks. Agreed. I wasn't worried about the SWR. Just curious about how three different meters at different points in the system give three different readings.

 

Common mode currents on the outside shield of the coax can cause SWR meters to display incorrect results.

 

The only time all meters connected in different locations will read nearly the same (except) for instrument calibration, is when the there is no reflected power, meaning the antenna end of the coax is terminated in the coax nominal impedance of 50 ohms.
Then there is no reflected power to measure at any point in the coax.
When there is reflected power, the coax becomes length sensitive or a Tuned line, intended or not.
Now any meter will read a different Ratio along the line.
This concept is being missed by those who ask why meter don't read the same.
Every meter placed anywhere in an 'unmatched' line will do the same.
Swap the meters around and you will still see the same readings in the same places in an unmatched line.

 

Well stated.

 

Thank you. So which meter reading, at which point in the system, is most meaningful as an indicator of the overall performance of your antenna system?