Saturday, January 16, 2016

Antenna Work

Did not happen today.

I dug out the bottom piece from the garage, only to find I've misplaced (what else is new......) the M6x8 bolt that holds it to the bottom of the antenna!

So, not having any metric bolts that size that fell "readily to hand", I made a run to Home Depot to get a replacement.

This issue with the antenna started sometime ago when I took down my Arrow Antenna GP52 6 Meter 1/4 wave vertical antenna and replaced it with my Comet GP-3 2M/70cm vertical so I could run the net from the shack, instead of using my Drake TR-270 which is in the living room on my wife's computer desk.

There were just too many times when people were over, or she was watching TV, and the "QRM" in the room was too high, and went out over-the-air, to properly conduct the net.

The GP-3 is a great little antenna, very rugged, and only 6' tall, meaning it doesn't really stand out in the neighborhood. The Drake is connected to it's little brother, the GP-1, and that's also an excellent antenna.

I'd been using the GP-3 at my apartment in Torrance before I moved here, and it worked extremely well. Back at the apartment it was on a 5' tripod, with a single 5' piece of mast, on top of a large two story building, with a clear view in all 360*. I could easily hit repeaters that were 70 miles away, and make reliable simplex contacts out to 30 miles.

Here at the house, it's on the same 5' tripod, ground mounted this time, with three 5' sections of mast, getting it well above the roof line with a clear view in all 360*. I'm even using the same length of coax, as it was "close enough" to being the right length, and was in excellent condition. The loss measurement for that piece of coax (Davis RF "Bury Flex") was as expected for the length, and the connectors were like new due to the fact I always properly weatherproof my connections on the outside ends.

I swept it with my Rig Expert AA-520 antenna analyzer, and the SWR was almost identical to the published curves for the antenna.

So I should be good to go, right?

Well, not really......The antenna just never "worked right", with receive being down from what it was in Torrance, although transmit seemed about the same. Now that's not really a good indicator, as I have a clear shot at the repeater, and can hit it almost "full quieting" with 5 Watts from my HT in the front yard. The problem came with simplex operation, when this higher gain (4.5dBi on 2M), physically higher antenna didn't even perform as well as the Comet GP-1 ("3dB" on 2M. No mention of it being dBi or dBd but I suspect dBi) on the other side of the house, and just barely clear of the roof line!

The only difference between the way it's mounted here at the house is when I swapped it out with the 6M antenna, I couldn't find the bottom "mount support pipe" piece, so I slipped it into a section of PVC conduit, and put it up.

This picture shows the metal piece, directly under the radials, at the bottom of the antenna:



And this is the actual item, measuring 245mm in length:



I'm not sure that this would be part of the antenna, or just a structurally sound piece to support the antenna. If it were actually part of the antenna, electrically speaking, I'd expect it to be held to the main part of the antenna with more than ONE bolt.

And since the radials are "too short" for doing much on 2M, I'm guessing that they're for the 70cm section of the antenna.

Guess I'll find out tomorrow when I put it back on the antenna!

11 comments:

  1. If YOU can't make it work, nobody can.

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  2. I'm not that good.....

    It worked fine before, so I just have to go back to the original configuration.

    I'll make a few before/after measurements to see if anything looks different.

    ReplyDelete
  3. 245mm? Hmmm. That's around 9 1/2 inches, which is about an eighth wave on 2m. That's certainly "electrically significant". Is that some sort of counterpoise that helps tune the antenna?

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  4. I think it's a bit "short" to make much difference, but I may very well be proved wrong.

    The thing is, the VSWR from my plots matches the published curves. If a significant chunk of the counterpoise was missing, I'd expect the VSWR to be high....

    ReplyDelete
    Replies
    1. The "works well on transmit but not receive" thing makes me think you have some losses there that aren't obvious. Antennas are pretty reciprocal in behavior. Like you say, the transmit load ought to be a good indicator. So the question is what makes transmit look good but receive worse?

      If you're doing a low band vertical, you really don't want 50 ohms; ideal is closer to 35. You can put down radials for a 1/4 wave vertical and make the VSWR worse. If you're seeing 50 ohms with only a few or no radials, the transmitter VSWR looks good but doesn't perform well in terms of getting power out. It may be more noticeable on receive.

      Something like that?

      Delete
  5. That antenna looks exactly the same as the Diamond X-50A. I think they are the same, just different names, both Japanese. I have an old hand me down Diamond I haven't used yet.
    Terry
    Fla.

    ReplyDelete
    Replies
    1. http://www.diamondantenna.net/x50a.html

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    2. Yep, Diamond and Comet antennas look, act, and "feel" almost the same.

      The only difference I've found is Diamond antennas cost a few $$ more!

      Delete
  6. SiG, I frankly don't know. From reciprocity, you'd sure think that degraded receive would show up in degraded transmit, and on the VSWR plots.

    I'm just about to make some measurements and then go "fix" the problem.

    If I can figure out how to post a pdf, I'll post the before and after plots. I might have to convert them to jpg or png so Blogger will accept them.

    Stand by for a new post in a couple of hours.....

    ReplyDelete
  7. Altitude plays into it. You were thirty feet up at the apartment, now you're 15 feet up... Do your horizon calcs and I'll bet you find that is the difference.

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    Replies
    1. Of course it does, but I've run the radio horizon profiles using SPLAT!, and they're not 30dB different, which is about what it seems like.

      I'm starting to suspect the coax may have gone south. It's 20 years old, and *looks* to be in great shape, but I'm just gonna have to run the loss measurements on it.

      It has a polyethylene outer jacket, and it's rated for direct burial, so being gently handled (no kinks or 'too tight' bends) and just laying on the roof for the last 20 years shouldn't have degraded it excessively....

      Delete

Keep it civil, please....