ad: elecraft

What is lost in an antenna tuner?

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

Thread Status:
Not open for further replies.
  1. KC2ZPK

    KC2ZPK Ham Member

    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

    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

    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

    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

    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

    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

    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

    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

    Can you recommend one?
     
  10. WB2WIK

    WB2WIK Premium Subscriber

    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.
     
  11. G3TXQ

    G3TXQ Ham Member

    "What is lost in an antenna tuner"?

    I once searched high and low for a 4BA spanner - eventually found that I'd left it inside my tuner ;)

    Steve G3TXQ
     
  12. KB4MB

    KB4MB Ham Member

    Of course if you switched to a balanced feedline and a balun right outside the shack, your losses would come way, way down (didn't see that post yet here). High SWR isn't nearly the factor on twinlead/ladderline as it is on coax, and it is dirt cheap!
     
  13. KC2ZPK

    KC2ZPK Ham Member

    Ladder line will not work at my location, due to Condo regulations, can't really have the antenna or coax, but they are mostly hidden.
    If I could run balanced line from the feed point to the base of the tree, about 20'-25', then coax to the window, that would work, but it woud be right up against the tree and has to cross about 20' feet of grass on the side yard. Currently the coax is barely buried in the grass.


    As a note, this is a temp install, I am house hunting as we speak, and hopefully this summer/fall will have a new QTH with NO HOA restrictions!
     
  14. KB4MB

    KB4MB Ham Member

    John,

    My advice is to not worry about it as long as your rig is happy, and since this is a temp situation, just live with it. 40m is the best band to have, and 15 should be working, but you could add a line isolator/choke balun at the rig and that might help. If you had the ability, adding an additional line underneath cut for 20m would be helpful, with say a six inch spacers so you would have three bands that would work pretty well.

    As for the real problem - the speakers, I did the same thing on my computer speakers which would come alive on 20m and what worked was turning the volume UP - this solved the problem. Or turn them off when you are operating :)
     
  15. AC2EU

    AC2EU XML Subscriber

    The tuner compensates for the lack of resonance in the antenna system, giving it more capacitance or inductance as needed for the operating condition. That being said, any energy (power) that is circulating in a tuner is not going to be radiating off the antenna element.
    Since electronic components are not perfect, there are also resistive losses in the form of heat in the reactive components of the tuner.

    The best solution, if possible, is having an antenna resonate on the band(s) of interest needing no tuner.
    The use of a tuner is automatically a loss scenario.
    The power meter may say that you are putting out 100W, but a good amount of that may be circulating around inside the tuner box.
     
  16. KB4MB

    KB4MB Ham Member

    A lot of us disagree with this statement - however, all would say that a good amount of energy is being lost in the feedline. The tuner will attribute to some lost, but not a huge amount (if we agree with Maxwell and his "Reflection" series, and I certainly do).
     
  17. AC2EU

    AC2EU XML Subscriber

    Yes, the velocity factor of the feed line is definitly a loss source, but that is just ANOTHER loss...
    Let's take what every Ham has complained about on Zed from time to time for an example; EXPLODING tuner caps. The reason this happens is when there is a very extreme compenastion where a large abount of energy is in the cap, not the antenna, causing it to seff destruct.

    When using a tuner, the antenna is nothing more than a reactive component in a resonant circuit, where the enegy is shared between the two.
     
  18. KC2ZPK

    KC2ZPK Ham Member

    I have another power meter down stream from the rig, and it is about 100W as well, maybe some loss, but not great.
     
  19. G3TXQ

    G3TXQ Ham Member

    I often read that statement, but it's a little misleading; there's not a great difference between how coax and ladderline handle high SWR as far as loss is concerned.

    For example, 100ft of Wireman 553 ladderline with a VSWR of 10:1 has a loss of 1.5dB at 30MHz; 100ft of LMR400 coax with the same VSWR has a loss of 2.4dB at 30MHz - less than a dB difference!

    What makes ladderline the better choice in a multiband doublet application is not that it handles high VSWR very much better than coax; it is that the highest VSWR on the ladderline is much lower. For example, the feedpoint impedance of a 132ft doublet might vary from 50Ω to 4000Ω depending on the band; on 50Ω coax those impedances represent VSWRs of 1:1 and 80:1, whereas on 450Ω ladderline they are 9:1 and 8.9:1.

    73,
    Steve G3TXQ
     
  20. VE3NLP

    VE3NLP Ham Member

    I have a SGC230, that I use on the end of a verable length of coax, so I don't run a power wire to the SGC. I put the SGC230 out were I want it, run how ever much coax it takes to reach the unit and it's good..How Do I power the SGC230 you ask? I have a small Solar cell charging a SLA battery, connected to the SGC230 to power it.. Works like a charm.. The Manual for the SGC says a run of wire over 50' can cause to much voltage drop.. May be that right maybe not. But have to have 20' to 100' or power wire on hand is too much work...The remote battery works like a charm.. Even at -40C the battery does not fail.
     
Thread Status:
Not open for further replies.

Share This Page

ad: vanity