What is lost in an antenna tuner?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC2ZPK, Jan 31, 2011.

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  1. KC2ZPK

    KC2ZPK Ham Member QRZ Page

    What is lost in an antenna tuner?

    Ok, say you have an 1/2 wave dipole on 40M, and it is cut for a good SWR @ 7.150 MHz. The build in tuner has no problem finding a match for the antenna. Now say I want to use it on some other band, 15M, and the tuner also finds a match, but it takes a while and I now have a little buzz in the computer speakers near the radio. Power output on the rigs meter is 100W for both. Is the antenna still radiating 100W on both bands? Would a 1/2 wave dipole cut for 15M be better? by how much. And how do I get rid of the buzz in the speakers? I already have ferrites on the cables going to the speakers.
     
  2. M0GVZ

    M0GVZ XML Subscriber QRZ Page

    No it isn't radiating 100W. Unless you're using a remote tuner at the feedpoint such as the SGC230, you will have losses in the co-ax from the high SWR as well as losses in the tuner. Say for example you're trying to run it at 21Mhz/15m feeding it with 100 feet of RG8 from the tuner to the antenna and the SWR without the tuner is 5:1. The losses on the co-ax will be 2.95dB so whilst your radio is outputting 100W then without even taking the losses in the tuner into account, only 50W is being radiated for the antenna.

    It is hard to say what the losses in the tuner are as there is a big difference between good ones and bad ones.


    If you cut the dipole for the bottom end of the 40m band it should tune up OK without a tuner as 15m/21Mhz is the third harmonic of 40m/7Mhz.
     
  3. W0BTU

    W0BTU Ham Member QRZ Page

    It would have to be a pretty darn bad tuner to have any losses that you need to worry about. The losses in coax are usual far greater than the losses in any antenna tuner.

    If the loss in the tuner were significant, then the tuner would get hot!
     
  4. G3TXQ

    G3TXQ Ham Member QRZ Page

    You said "some other band", and then quoted 15m. A 40m dipole exhibits a moderate impedance on 15m because it's close to being 3/4 wavelength long; that keeps the losses in the coax manageable.

    But if you operated a 40m dipole on 20m you might expect about 10dB loss in 100ft of RG213 feedline and - depending on coax length - over 30% power lost in the tuner.

    Steve G3TXQ
     
  5. W5DXP

    W5DXP Ham Member QRZ Page

    There's really not enough information to respond. EZNEC says a 66 foot dipole at 40' used on 15m will have a feedpoint impedance of 102-j211 ohms and an SWR on the coax of 11.1:1. That SWR will result in a loss of 65% (4.5 dB) in 100 ft of RG8x coax. The loss in the tuner is probably about 5%, i.e. negligible. How long and what kind of coax are you using? Here's a transmission line loss calculator:

    http://www.vk1od.net/calc/tl/tllc.php
     
  6. KC2ZPK

    KC2ZPK Ham Member QRZ Page

    Sorry I should have been clearer.
    I was ignoring the effect SWR has on coax loss, but I see that would be incorrect. This is what I am getting at. I have a 40M diploe and 80M dipole connected at a common feedpoint , see this thread for more background http://forums.qrz.com/showthread.php?282690-Opinions-and-comments-on-my-40-80-Dipole , and the tuner matches on both 40 and 80 meters. I have found it also matches on 30M, but I noticed from the psk reporter that I wasn't getting out as far as 40M. I was wondering if a bad mismatch caused high losses in the tuner, but it looks like the losses are in the coax. Would this be the reason for remote tuners?... You lower the SWR on the feedline to minimize losses? Anyway, back to my dilema. I would like to possabily redo this antena for coverage on more bands. As I mentioned in the other thread, it has to be stealth. I live in a Condo, and the "condo-cops" would turn me in to the board in a heartbeat. I am using a very small PVC cap (1 1/2" DIA) and a plug to house the coax connector and the solder connections for the two dipoles. Over all it is maybe 3 inches long, painted to match the trees. :) it is hardly visable at 25' up, once the leaves come in you will have to put effort into finding it. If I can get the same conceilment from another antenna that has better band coverage, I am all for that. Trap dipoles won't work because the traps will be visable, and ladder line does not work as it would be visable hanging out my window to the trees.
     
  7. W5DXP

    W5DXP Ham Member QRZ Page

    To get the dipole to work on 15m, I would increase the 40m dipole length to 68.5 feet. It will still probably work well on 40m.
     
  8. M0GVZ

    M0GVZ XML Subscriber QRZ Page

    Yes thats exactly right. I constantly fail to understand why people are happy to use an ATU at the transceiver end of the co-ax run and accept the massive losses in the co-ax where for the sake of running a +12vdc and 0v wire alongside the co-ax they can have an antenna coupler at or very near the feedpoint putting as much power as possible into the radiating element.
     
  9. KC2ZPK

    KC2ZPK Ham Member QRZ Page

    Can you recommend one?
     
  10. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    LDG tuners are fine but not weatherproof, so you have to protect it somehow. SGC makes weather resistant tuners. MFJ has a fairly new one on the market which is also weather resistant.

    I used an SGC-230 (old model) outside for many years, at the base of a tree about 200' from the house, protected by nothing more than a cheap Igloo ice cooler with the lid closed and sealed by refrigerator tape (which is extremely weatherproof), and the cables going in and out of a hole in the side of the cooler protected from the WX by a PVC elbow mounted at the hole (facing "down," so the cables came in at ground level and went "up" into the elbow and then into the cooler). $19.95 solution, lasted several years.
     
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