Ask Dave – Episode 1 – Antenna Analyzers

Posted on June 11, 2015 by

New video series! Ask a question about ham radio and I’ll try to answer it in a video. Click here for the form. And, I love feedback, so be sure to comment either here or on YouTube!

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6 Responses to Ask Dave – Episode 1 – Antenna Analyzers

  1. Dave says:
    January 7, 2016 at 9:07 pm

    Hi Phil, sounds like your club members have a great antenna idea. And, yes, a gamma match is a great way to go. It is also true that if you have a feedline that is an odd multiple of a quarter wavelength, the impedance you see at the antenna will be the same impedance offered to the radio (but will be affected by ohmic losses in the cable itself). However…all that is true only at a single frequency! And the 50 MHz band is pretty wide. So a feedline that’s a perfect odd multiple of a quarter wavelength at one frequency will not be as you tune away from the design frequency. Your friends are right that a simple SWR meter will not tell you when the match is 50 ohms. Borrowing an antenna analyzer will solve that problem. However…when the resistive part of the match is 50 ohms, there’s no guarantee that the reactive part will be zero! A gamma match also involves a capacitor in some shape or form—when you adjust this you may get it very close to a perfect match. But then…if your antenna is a perfect 50-ohm resistive antenna with no reactive component, you can feed it with any length of transmission line and the transmitter will see 50 ohms! I recommend setting up the antenna so it’s a perfect match at the frequency your club uses the most, or your favorite DX or CW frequency. Judicious adjustment both of the capacitor (often simply a copper or aluminum tube with a rod inside and some sort of dielectric—you adjust it by moving the rod in and out) as well as where the gamma match attaches to the loop will get you very close to 50 ohms. Then use your SWR meter to see how much bandwidth the antenna can cover between the two 2:1 SWR points—I suspect it won’t be the whole band. Good luck, and send me a photo of the result! 73, Dave, KEØOG

  2. phil says:
    January 7, 2016 at 12:58 pm

    Hi Dave liked this video but, can you help? Im thinking of building a halo for 50mhz & here in uk we have 70mhz (for horizontal omni directional) & other bands & using a gamma match for connection. I was at a club many miles away & the way this halo i see was set up the performance was amazing. Now apparently the reason it was so good was because the shorting bar from the gamma to the main halfwave section was at the 50ohm point. I was told that once the 50 ohm point was found then you adjusted the gamma tuning to bring the antenna into tune. Now i mentioned to the guy that i have a analyzer the MFJ209 but apparently it wont be of any use as it doesn’t show impedance & i was told i need one to find the 50 ohm point first. Does this sound correct to you? If so how do i use the meter you have to find the 50 ohm point? One other thing regards the length of coax i believe multiples of half wave plus 1 /4 on end is best for any mono band antenna as this gives maximum current at the feed of the antenna (taking VF into account) 23ft you used unless ive miscalculated isn’t ideal for the 6meter ?

  3. Richard says:
    June 12, 2015 at 11:15 am

    Hi Dave, “Ask Dave ” is a great idea. The first session on SWR was certainly a perfect starting point because there is considerable misunderstanding among many new hams on this very subject…and even some more experienced hams can gain a better understanding. Talking about an issue is one thing, but actually demonstrating the results with explanation adds a whole new dimension to learning. Many of us learn by seeing/doing as opposed to reading and rereading manuals. Thank you Dave !!
    And I love that photo at the end of Silverton !! My wife and I rode the D&S in the spring of 2010, and it was one of the very best scenic areas that we have ever visited. Had I known of you back then I would have visited you as we did have a rented car and came very close to your home on the million dollar highway…550.
    73, Dick / W1REJ

  4. Gregory Hill says:
    June 12, 2015 at 10:08 am

    Nice job!

  5. Art Womer says:
    June 12, 2015 at 6:20 am

    A big THANKS, Dave! I’m planning to build a 6m antenna and this is a great help! Watched all the EXTRA videos and their also a great help! Keep up the good work.

    73 from Hermitage TN
    Art Womer
    KI4OYV

  6. Chris Boehm says:
    June 12, 2015 at 4:02 am

    Great idea for a series Dave! It’s hard to believe nobody has done this before now! You are a true pioneer. Keep up the good work. Another title for the series could have been Everybody’s Elmer. A video on Elmering might be a good topic also.

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