Tuner loss, line length and a field strength meter

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KD7MW, Apr 10, 2021.

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  1. KD7MW

    KD7MW XML Subscriber QRZ Page

    How far away from an HF antenna do I have to be before I can trust a simple diode-based field strength (FS) meter to make meaningful relative measurements of the antenna's output? I've been doing some tests with my multi-band dipoles fed with ladder line. I want to see if varying the line length affects antenna tuner loss enough to bother about it. I don't own an RF ammeter, so I thought perhaps a field strength meter would tell me what I want to know.

    I used "patch cables" of ladder line with banana plugs and jacks to vary the line length. I had patches of 3, 7, and 14 feet, plus one combination of 21 feet for 80 meters. I did a frequency sweep with a RigExpert AA-54 analyzer and software to get the R and X values every 100 kHz, for each length of line on each ham band. I used this T-tuner simulator to arrive at the line length with the lowest loss figures:
    http://fermi.la.asu.edu/w9cf/tuner/tuner.html.

    I connected the FS meter's antenna to a curtain rod on the opposite side of my house from my radio room, much nearer to the antennas in the back yard than to the radio. But we are talking near field, not far field. I ran through my radio's RF power control, found that each tick on the FS meter represented about 2% (~2 watts) difference. Then, on each band, I set the FS meter to about 3/4 scale for one tuner/line length reading, and did not touch it for any of the other combinations of tuner and line length. I also sat in exactly the same place while taking each reading, to avoid changes due to proximity effects.

    In some cases, I got results I expected. But not always. On a couple of bands, the line extension that gave me "good" R and X values gave me worse FS readings. And sometimes my MFJ 974B balanced T tuner actually gave me better FS readings than the same length of line with my Ten Tec 238A plus 1:1 external Guanella choke balun. This despite ARRL reviews that show the Ten Tec to be far superior to the MFJ at low to medium impedances, except on 10 meters. These results were repeatable. But I wonder, am I measuring what I think I'm measuring?

    The good news is that in no case were the differences more than 22% power with a 100 watt signal, and usually much less. 20% is about 1 dB, or 1/3 of an S unit, which is not going to make much difference at the other end of a QSO. So I'm inclined to just use my feedlines as-is unless a particular band can't be tuned to 1:1.
     
  2. AA5MT

    AA5MT Ham Member QRZ Page

    You might try one of the online receivers. They are in many locations across the world. They show the s meter on most of them. This is the cheapest setup, and the most accurate. You can turn your antenna and plot the pattern. I listen to radio at work like this, as I do not have a radio there. I use my PC, but I can also use my phone with ear buds for more privacy.

    Google na5b in DC.

    Tom
     
    KU3X likes this.
  3. KM3F

    KM3F Ham Member QRZ Page

    As I understand you<< ,you have several things to account for making the reading near useless for being accurate.
    The major one is, what antenna are you using for the tests. Is it multi band?
    Next; changing bands changes the pattern position and strength the FS meter will see. This gets into antenna theory for the causes.
    Next, changing the Ladder line length changes the match and (may affect) power transfer.
    Next, Ladder line has such low loss on HF bands as to make it almost impossible to see any actual loss effects included in overall testing. Note that loss is different than what miss match it may cause by changing lengths even though added length does add a 'very small' amount of extra loss, as well.
    Next tuner losses might be able to be seen but also get lost/ included in the changing setup.
    .
    Look up March/April issue ARRL QEX mag front cover and article on tuner loss measurements.
    Bottom line is you have to fix each test with with one change then return back and make a different change to see the individual effects. A/B testing.
    That's a lot of changing but would be much more accurate.
    Respectfully, you came with the question because it did not make much sense do to all the different effects as above plus others that may be involved.
    As in everything being tested, if more than one change is made at the same time, the result certainly can change but "WHO DONE IT"? the detective asks.
    Good that you want to know and learn.
    Good luck.
     
  4. KM3F

    KM3F Ham Member QRZ Page

    Checking a distant RX location will certainly take into account all the test variations you make including propagation as long at they are made in a short time span. This may not be possible, physically.
    Otherwise the checks would need to be made over a longer period and an averaged to see if any specific setup favors >>for that path<< , band and time of day.
    Good luck.
     
  5. KD7MW

    KD7MW XML Subscriber QRZ Page

    Thank you both for the replies. Tom, AA5MT, thanks, I’d forgotten about those remote receivers. That would be the best test. KM3F, these are simple non-resonant dipoles fed with ladder line. All comparisons were made between different feeder lengths on the same band, with the tuner properly adjusted.

    The issue is that some R,X combinations can cause loss in the *tuner* of up to 50%, particularly on the low bands. All I’m after here is to find out, for example, if on 80 meters, I really should patch in 14 extra feet of line to avoid tuner loss that makes for a noticeably weaker signal on the other end. Or if on, say, 17 meters, Tuner A eats significantly more power than Tuner B. Or if it really doesn’t matter much, so there’s no need to hassle with different feeder lengths.
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    With every kind of tuner I can think of, all the loss is in the inductor; so usually if you have three adjustable elements (often T-matches are used with two capacitors and one inductor), the "lowest loss" setting is the one that uses the least inductance.:)
     
  7. KD7MW

    KD7MW XML Subscriber QRZ Page

    Thanks, Steve. Yes, I know that rule of thumb.

    I've tried the WebSDR server in northern Utah. Problem is that today's daytime slow QSB totals about 10 dB, which is far greater than the differences I am trying to measure. I tried a couple of different setups on 40m and 30m, and looked for signal peaks over a minute or two. The various combinations of tuner and additional line vs. none all came out within a dB of each other. Cutting my power to 50% did indeed lower my received signal about 3 dB, as it should. I'll try again at night to check 80m, where losses might be the greatest. What I really need is a friend nearby, within ground wave coverage, to cut out the QSB issues.

    What I've seen so far does indicate that the differences are probably not significant to an operator on the other end of a QSO. So it's probably not worth patching in various lengths of line for each band just to reduce losses a few percent. Now, on 80 or 40, if I can bring losses down from ~30% to the single digits, that might be worth it
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    A slick indicator of very far-field signal strength is RBN. It will catch you if you call CQ or TEST on CW and has dozens and dozens of skimmers all over the world that report back in near-real time on their website with signal strength (and they also report code speed, which seems frivolous, but there it is).

    If I want to compare two antennas for the same band, I send TEST de WB2WIK a few times with one and rapidly switch to the other and do the same thing, then go back and forth doing that maybe ten times to average out the QSB. But that can all be done in two minutes. And it's interesting because one antenna may be way stronger in Maryland but way weaker in Germany, then another antenna may be way stronger in Germany and way weaker in Maryland...all within the same minute.

    That evaluates a lot of stuff all at once: Not so much "antenna efficiency" but more about takeoff angles and how the F layer is doing at the moment.

    Unless your antenna is vertical, 80m groundwave could be really disappointing.:p But still may be a valid data point.
     
  9. W4HWD

    W4HWD Premium Subscriber QRZ Page

    "How far away from an HF antenna do I have to be before I can trust a simple diode-based field strength (FS) meter to make meaningful relative measurements of the antenna's output?"

    The FCC uses a distance of 1km to measure field strength of AM broadcast stations. The units are millivolts/meter at 1km. This method would be useful for 160 and 80 during the day, and higher bands only if they are closed; otherwise, propagation will skew your measurements.
     
  10. W4KJG

    W4KJG Subscriber QRZ Page

    If you can run low enough power, try WSPR with the transmit intervals set fairly close together. But, as WIK says, try RBN too. Operating FT8 and watching your signal intercepts on PSKreporter will also tell you quite a bit.
     

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