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Is 450 ohm twinlead better

Discussion in 'General Technical Questions and Answers' started by K0CMH, Mar 12, 2008.

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  1. W5DXP

    W5DXP Ham Member QRZ Page

    If the tuner will match a 40m dipole used on 160m..... Operate?
     
  2. W4DNR

    W4DNR Ham Member QRZ Page


    Don't blame an improperly designed antenna on the feedline.

    To answer your question... Yes, If all you have is a 40 meter dipole
    and you want to try 160, and the tuner will present an acceptable match to the transmitter, why not ?? It might not be the most efficient setup, but
    several steps above ( for instance ) a 160 meter mobile whip.

    Don WA4NPL
     
  3. KA0GKT

    KA0GKT Ham Member QRZ Page


    One thing which hasn’t been mentioned is that if the impedance of the terminating load is the same as the characteristic impedance of the transmission line (the line is matched), the impedance presented to the transmitter will be the impedance of the load. No ½-wave lengths are necessary under those circumstances as any length will match.

    So, if you are having problems with a mobile antenna to which you added transmission line, you may have problems other than the simple added length.

    1. The antenna may be presenting termination impedance far removed from the characteristic impedance of the transmission line. (i.e.: it’s not a 50-Ohm antenna)

    2. The connectors used to “splice” the additional length of transmission line may not be properly attached to the transmission line.

    3. VSWR generally doesn’t affect receive in modern receivers. The RF amplifier is generally an FET (Field Effect Transistor) type device with relatively high input impedance. If the addition of a VSWR meter and jumper makes a difference on receive, you may have connectorization problems somewhere.


    Yes, it is a great experiment, go for it! Remember that most commercially made antennas are designed today to present a nominal 50-Ohm termination to the transmission line when resonant. If you are do not get a usable VSWR on a commercially made antenna, some adjustment may be in order. There are some antennas on the market today which are really designed for land mobile (business or public safety) service, and are not tuned to amateur frequencies out of the box. Most times, these have cutting charts to give the end-user a starting point for resonating the antenna.

    Good luck with your experiment. If you are a member of an ARC, check with other members. Some clubs have access to test equipment the average Amateur cannot afford. Looking at your antenna with a Vector Network Analyzer or a spectrum analyzer, tracking generator and reflected power coupler can be really educational.
     
  4. W5ALT

    W5ALT Ham Member QRZ Page

    What seems to be missing, unless I overlooked something in all the replies, is your analysis of the impedances involved. You can't figure impedance matches or mismatches by starting at the transmitter, unless everything is the same impedance. It's a common mistake.

    To analyze impedances, you have to start at the antenna, which is a load for the feedline. If you have a mismatch there, then the other end of the feedline will show some impedance that depends on the antenna load impedance, the length and the characteristics of the transmission line. That is the impedance that is presented to the transmitter, not the characteristic impedance of the transmission line itself.

    So, if you have an unknown antenna impedance and an unknown length of transmission line, you have an unknown impedance at the TX end. Whether or not a 4:1 balun would help is also unknown, since if you don't know what the impedance is, you likely don't know what 1/4th of it is, either. And since you don't know the impedance, whether or not it matches one characteristic impedance or another is also unknown.

    The only real solution is to try it. If you can load your transmitter into whatever you have and don't burn anything with too much power, you're good to go. The real reason to use ladder line, TV twin lead or coax is a trade-off between convenience, power handling, and losses. If you can handle the power and the transmission line runs and connections, then the losses might figure in, but on HF the difference in losses between twin lead and ladder line isn't very important. As others have pointed out, power handling may well be a limit.

    73,
    Walt, W5ALT
     
  5. AG3Y

    AG3Y Guest

    Before this thread got into transmission line theory, someone mentioned that they did not like to use 450 ohm window line because it was "unstable".

    My answer would be to make sure you put some twists into the feedline, so the wind doesn't get a chance to hit it flat on, and whip it around !

    I have been using window line with great success here, ( including matching impedances with it ) and that little trick has resulted in much joy for me.

    Twist it about a turn every 8 to 12 inches, and you won't have any wind problems !

    73, Jim
     
  6. KA0GKT

    KA0GKT Ham Member QRZ Page



    I can predict a follow-up question (for someone at least)

    "Then how do they test commercial antennas at to top of tall towers?"


    With test equipment like a Vector Network Analyzer, the transmission line can be factored out of the equation. It is quite easy with newer VNAs because the adustments are all done automatically by the instrument. The end of the transmission line where it connects to the antenna is terminated in a calibrated load, calibrated open and a calibrated short. The VNA calculates the charecteristics of the transmission line and then factors the line's effects out of the readings allowing the instrument to display just the antenna's response. No wonder a good VNA can run as much as 50 Kilobucks!
     
  7. W4DNR

    W4DNR Ham Member QRZ Page


    Steve,

    I have a Channel 31 UHF TV antenna at 1000 feet up.

    The VNA will work fine , but you still have to send a crew up to connect the treminating resistor , the short, and then back to the antenna.

    Either way ( measuring at the antenna or at the transmitter end ) someone still has to get to the antenna feedpoint. Ya can't win. LOL !!!

    WA4NPL Don
     
  8. N6YG

    N6YG Ham Member QRZ Page

    Hi Jim

    I can't believe this made it all the way to page 4 and nobody other then you mentioned twisting the feed line. It makes me wonder how many people actually pay attention to whats going on with their station.

    By the way I was the one that mentioned instability. Its funny on a windy day if you tx a steady carrier you can actually see the SWR fluctuate, sometimes drastically. The 300 ohm TV twinlead does not do this as much, it just has issues when it gets wet.

    Twisting the feed line does indeed fix the wind induced problems. Thanks for mentioning it.

    Talk about twisted feed line. A while back I was testing a commercially built doublet that comes with 600 ohm parallel conductor. We were at the park had the antenna strung between 2 trees at about 75 feet and it was a some what windy day. Somehow the wind whipped the antenna so hard that it broke a spreader loose. As a result the feed line twisted and shorted. My son was calling CQ and I was about 3 feet away when I heard the sounds of arcing. I thought the tuner is arcing?? Sure enough that's exactly what was happening. When I got the antenna down for inspection I realized that the feed line was fused together solid. Amazing what 600 watts can do.. Yeah I know I said I like working portable, I never said I like QRP :D :D :D :D
     
    Last edited: Mar 16, 2008
  9. KA0GKT

    KA0GKT Ham Member QRZ Page

    No Kidding, Don!

    My tower and antenna is only a smidge under 300' AGL, but the drive up to 8535'AMSL takes a couple of hours if you don't need to take the Thiokol Spryte the last three miles to the site...and those tower crews don't go on the clock when the get to the base of the tower!

    I have a Channel 6 batwing atop a channel 30 panel antenna for DTV. I am looking forward to pulling the switch on the Channel six in eleven months.

    So, do you use the VNA as an aid in tuning the Klystrons? Our Advantest VNA does a great job driving our IOTs and makes the whole tuning process...well, not a joy, but not a pain either!

    I'm planning a little antenna work on the 215.6-degree radiator for our AM station on 1550. This will be the first time I get a chance to try the VNA on that antenna, I plan to compare results between the VNA, the Potomac bridge and the old General Radio hot bridge. The KUAZ antenna is the last of our antennas to be documented with the VNA. So far, I've taken readings on all of the STLs, both full power FMs, six FM and one TV translator. It's best to do this kind of work between the winter rains and the Summer heat here in the Sonoran Desert!
     
    Last edited: Mar 16, 2008
  10. KC4RAN

    KC4RAN Ham Member QRZ Page

    I thought I should bring this older thread back from the depths. I learned quite a bit reading through this one...
     
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