Using Field Strength fs meter with Ant Tuner / SWR

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Using Field Strength fs meter with Ant Tuner / SWR	Reply
by N3SDO on December 19, 2007	Mail this to a friend!
Can somebody enlighten me to what is happening here, and the best way to optimize a tuner? Im Running an Inverted Vee, cut for 40 meters, with a low end MFJ tuner. I have noticed that when I tune Vee up on 75m, there are multiple combinations of Input C + L + Output C that will produce a 1-1.2 SWR at the rig. I hooked up a HomeBrew relative FS Field Strength meter to a short vhf ant, and I can peg the meter on 40 ssb. I was suprised to discover that it dident register anything on some tuner combinations on 75m, even though the swr was good. Doing some twiddling, I can tune for max FS indication looking only at the FS meter, ignoring SWR, and the SWR is tolerable, 1.5 to 2. I can fine tune the swr down lower, but lose a tad of field strength. Is this from losses in the tuner, am I mis-adjusting the tuner, is my feedline / balun converting my RF to heat at some settings? Is this normal?

RE: Using Field Strength fs meter with Ant Tuner /	Reply
by N3OX on December 19, 2007	Mail this to a friend!
"Doing some twiddling, I can tune for max FS indication looking only at the FS meter, ignoring SWR, and the SWR is tolerable, 1.5 to 2. I can fine tune the swr down lower, but lose a tad of field strength. " I assume you're using ladderline to the antenna? I think what's happening is this, based on the fact that you see no indication at some settings on 80m and plenty of indication on 40m: I think when you tune for maximum field strength, you're actually tuning for maximum feedline radiation! - - - - - - I think there's going to be a very slight difference in the common mode current (causes line radiation) that flows on your feedline depending on the exact settings of the tuner components. I think that's what you're tweaking ... If you want to use the field strength meter it should really be out near the antenna... it sounds to me like it's probably in the shack on top of the tuner or some such? A field strength meter is a much more sensitive detector of RF-in-the-shack than other indicators like RF feedback into the rig. A field strength meter is a good tool but if it's very near a radiating feedline that's what it's going to measure, not the radiation from the antenna that's much weaker in the shack. 73, Dan

RE: Using Field Strength fs meter with Ant Tuner /	Reply
by WB2WIK on December 19, 2007	Mail this to a friend!
I agree with the previous post. If you're using coax to load a 40m dipole on 75m, the SWR on the line itself will be awful no matter what you do with the tuner; this is a combination that really doesn't work.

RE: Using Field Strength fs meter with Ant Tuner /	Reply
by AD5X on December 19, 2007	Mail this to a friend!
A C-L-C "T" tuner has a very wide matching range, but it also has several different component settings that will get you a good SWR as you've seen. However, some settings can result in more or less losses than other settings due to the resulting tuner matching network Q. This means that a lot of your power can be burned up in the tuner instead of going to the antenna if you choose the wong combination. Incidentally, an L-tuner normally has just one matching solution. But then you don't have as much of a matching range as you do with the T-tuner. Phil - AD5X

RE: Using Field Strength fs meter with Ant Tuner /	Reply
by NH7O on December 19, 2007	Mail this to a friend!
Here is a link which expands on the above: http://ham.srsab.se/pdf/test_MFJ989.pdf This shows that it is possible to adjust a tee match such that extremely high loss results.

RE: Using Field Strength fs meter with Ant Tuner /	Reply
by AB3CX on December 21, 2007	Mail this to a friend!
The rig wants to see 50 ohms impedance. At an SWR of 2:1, that could be an impedance of 25 ohms or 100 ohms. At an SWR of 1.2:1 that could be 41 ohms or 60 ohms. So if the tuner can transform the antenna load to either one of those two impedances, those will be the SWRs. What the SWR meter does not tell you is the phase angles and therefore what the currents and voltages will be on the radiating elements at those settings. The radiation resistance and antenna efficiency is likely quite different on the short antenna with the various settings of the tuner

RE: Using Field Strength fs meter with Ant Tuner /	Reply
by N3SDO on December 23, 2007	Mail this to a friend!
N3OX, Ladderline to ant, nope, coax fed with 4 ferrite beads for decoupling. FS meter is 6 feet from rig, but feed lines do go parallel about 4 feet apart for a couple of feet. Im going to do some up-close feedline sniffing to look into the feedline radiating. WB2WIK, Coax feed, um, well, yeah. Scrapped thin ethernet teflon plenum coax, like RG58, except free.. I don't have any 600 ohm ladder line, but got about 30' of 300 ohm. Should be better then RG58/thinnet. AD5X, Internal Tuner loss, yes thats my original suspicion. NH7O, Thanks for link, very informative! Im sure my MFJ-941b rotary switched inductor is not as good as the roller inductor tuner tested. AB3CX, If I understand the impedance, a 40m ant when run on 80m is a very low impedance to produce the high swr. Thankyou to all: I suspect the combination of tuner loss matching a low impedance, plus thin coax loss plus is losing a good portion of my signal. I lack the yard space to "properly" install a full size 80m ant, but might be able to mid-load (trap?) the elements and feed it with 300 ohm flat twinlead.

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