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**Antenna SWR Problem**

Discussion in 'General Technical Questions and Answers' started by W0LC, Nov 27, 2003.

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  1. W0LC

    W0LC Ham Member QRZ Page

    I am experiencing a unique and new problem that I have never encountered before.

    I erected an 80 meter double bazooka and when hoisted up to around 48 feet, 1 foot from the tower, the SWR will not resonate. The ends are 15-20 feet off the ground. The tower is an aluminum guyed (with kevlar) tower with hf yagi, 6 meter yagi atop.

    Fed with new 100 foot RG-8 coax, I cannot get the stupid thing to resonate.

    However when the apex is resting on a 6 foot step ladder, with ends almost on the ground, the antenna shows 200-300 khz bandwidth with very lower SWR which it is suppose to do.

    I took it down and put up an 135 dipole/inverted Vee fed with 450 ohm ladder line. Pretty much the same problem, no resonant spot on the 75 meter band.

    Anyone have any ideas what I might be seeing? This is something new to all my years of putting of wires.

    Sure has me stumped. The distance from the tower (apex) shouldn't be an issue, and the tower grounding shouldn't affect it either.

    Any ideas of where or what to look for?

    Thanks for reading this!
     
  2. AG3Y

    AG3Y Guest

    How tall is the tower ? about a 1/4 wavelength on 80 meters? Is it a brand new tower ? Have you had success with this tower in the past on 80, or is this your first attempt at this?

    Please give us some specifics. I wonder if the tower length is interacting with your antennas on 80 meters ?

    73 from Jim and Happy Thanksgiving!
     
  3. W0LC

    W0LC Ham Member QRZ Page

    Sorry. When one types in info, they tend to leave out some. Hi.

    Tower is 2 years old, Glen Martin tower with hazer.
    50 Foot tall tower, with apex for hanging antennas about 46-48 feet. Sloper is afixed to tower top and seems to work fine as it did in the previous QTH.

    Sloper is about 90 degrees in respect to any hanging wire antenna.

    Was running on the bazooka, 100 feet of new cable experts RG-8X with silver connectors (yes, tried different coaxs too). Dipole/inverted Vee is using 46 feet of 450 ohm line with balun box on overhang of the house with coax into shack.

    ANything else, just let me know!

    Sorry about leaving out some info!

    Chris
     
  4. Guest

    Guest Guest

    Chris,
    You havn't mentioned the SWR across the band, but I'm wondering if you used lenght=492/Fmcs instead of lenght=468/Fmcs.

    492 is for free space 1/2 wavelenght, 468 is for actual wire in the real world.

    You said a 135 (I assumed feet) dipole. That would be resonant at about 3.467 mcs. a bit below the 80 meter band.

    73, Marty WB2RJR/7
     
  5. W8JI

    W8JI Ham Member QRZ Page

    </span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (wc5rr @ Nov. 27 2003,09:35)</td></tr><tr><td id="QUOTE">I am experiencing a unique and new problem that I have never encountered before.  

    I erected an 80 meter double bazooka and when hoisted up to around 48 feet, 1 foot from the tower, the SWR will not resonate.  The ends are 15-20 feet off the ground.  The tower is an aluminum guyed (with kevlar) tower with hf yagi, 6 meter yagi atop.

    Fed with new 100 foot RG-8 coax, I cannot get the stupid thing to resonate.  

    However when the apex is resting on a 6 foot step ladder, with ends almost on the ground, the antenna shows 200-300 khz bandwidth with very lower SWR which it is suppose to do.

    I took it down and put up an 135 dipole/inverted Vee fed with 450 ohm ladder line.  Pretty much the same problem, no resonant spot on the 75 meter band.

    Anyone have any ideas what I might be seeing?  This is something new to all my years of putting of wires.

    Sure has me stumped.  The distance from the tower (apex) shouldn't be an issue, and the tower grounding shouldn't affect it either.  

    Any ideas of where or what to look for?

    Thanks for reading this![/QUOTE]<span id='postcolor'>
    You won't get a low SWR with a dipole fed with 1/4 wl of open wire line, so forget about that unless you use a tuner!!!! Most baluns won't even properly handle the impedance that antenna would present.

    As for the coaxial dipole, I agree with others who replied the length might be wrong. I doubt it is tower to dipole interaction, but then it sounds like you do have some other stuff hanging around like a &quot;sloper&quot; antenna so there is a remote chance.

    I really think the dipole is constructed wrong, or has a bad connection.

    Why a coaxial dipole anyway? Why not just a regular old dipole?

    73 Tom
     
  6. W0LC

    W0LC Ham Member QRZ Page

    Coaxial dipoles usually have a quieter recieve and a slightly better gain then the standard dipole. Plus, I had one that I inherited laying around.

    Did put up a 75 meter dipole and found that the bandwidth was only 150 khz. It was of course fed with coax and not ladder line.

    Trying to use that for multi-band use is okay when using a tuner, but the SWR is quite high on other bands.

    Was trying to get something that would work adequately for 75 and 40, adn the WARC bands. Other then a trap type antenna or multiple wires, the tuner method is what many use.
     
  7. W8JI

    W8JI Ham Member QRZ Page

    </span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (wc5rr @ Dec. 01 2003,05:42)</td></tr><tr><td id="QUOTE">Coaxial dipoles usually have a quieter recieve and a slightly better gain then the standard dipole.  Plus, I had one that I inherited laying around.

    Did put up a 75 meter dipole and found that the bandwidth was only 150 khz. It was of course fed with coax and not ladder line.  

    Trying to use that for multi-band use is okay when using a tuner, but the SWR is quite high on other bands.

    Was trying to get something that would work adequately for 75 and 40, adn the WARC bands.  Other then a trap type antenna or multiple wires, the tuner method is what many use.[/QUOTE]<span id='postcolor'>
    In the 1970's a fellow working for me made the same claims. Part of his sales pitch was they had gain, were quieter, and had more BW.

    We measured an antenna, removed it, and installed another antenna in exactly the same place (in the clear) and found it had no more BW and no more gain than a regular dipole from thick wire. Those were his home made Bazookas that he sold at Hamfests.

    This debate came up again on Towertalk around year 2000 (25 years later!!&#33[​IMG], and someone actually sent me an IAC Bazooka to test. I measured that IAC Bazooka antenna here it was identical in noise and bandwidth to a regular dipole and had just slightly less gain than a regular dipole made from the same gauge wire.

    If you read the ARRL Handbook sections about HF antennas, you'll see they say the same thing as I found. As a matter of fact, my measurements of an actual antenna were within a few percent of what the ARRL Handbook claimed through calculations.

    73 Tom
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd just use larger coax and use the dipole or coaxial dipole, or whatever.

    I have a similar problem here at home: 54' tower with 80m inverted vee supported by 5' long standoff arm (to hold it away from the tower) and tied to two supports such that each end is about 15' above ground. Tower cranked up, antenna shows no dip anywhere in the 3.5 - 4.0 MHz band, SWR is about 4 or 5 to 1 across the band; however, it works fine, hears well, gets out as well as it hears and I can copy the W1AW code practice on 80m CW as soon as the sun sets (3000 miles away). It just doesn't match well at all. In my case, I'm feeding with 160' of RG214/U, with the feedline bisecting the enclosed angle of the inverted vee. Tried with or without a current balun, doesn't matter.

    However, if I crank the tower down to about 40' so the legs of the antenna are &quot;droopy,&quot; it loads up much better, showing a nice, smooth dip at about 3750 kHz, where it dips to about 1.8:1 or so.

    Crank the tower back up, back to same problem as indicated above.

    I decided to forget about it. My amp's pi-L can handle the load just fine, and so can the autotuner in my transceiver. 160' of RG214/U with a 5:1 SWR at 4 MHz doesn't have much loss, so I stopped making antenna adjustments. Can still get through first or second call to DX, usually.

    I still don't know why my 80m inverted vee won't resonate with the tower cranked up! Possibly it's an imbalance in the antenna-to-earth or antenna-to-surroundings that becomes worse when the legs are at 45 degrees (to the tower, and to ground) than it is when they're more horizontal.

    At this point, I don't give a darn!

    WB2WIK/6
     
  9. N8CPA

    N8CPA Ham Member QRZ Page

    You didn't mention how far the end supports are from the tower--unless I missed it. If the leg to leg apex angle is less than 90 degrees--I can't remember the term for the condition--self parasitic? Currents in the legs cancelling each other out?

    Where did I put my Antenna book? [​IMG]

    !!
     
  10. W8JI

    W8JI Ham Member QRZ Page

    </span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (WB2WIK @ Dec. 01 2003,09:51)</td></tr><tr><td id="QUOTE">I'd just use larger coax and use the dipole or coaxial dipole, or whatever.

    I have a similar problem here at home: 54' tower with 80m inverted vee supported by 5' long standoff arm (to hold it away from the tower) and tied to two supports such that each end is about 15' above ground.  Tower cranked up, antenna shows no dip anywhere in the 3.5 - 4.0 MHz band, SWR is about 4 or 5 to 1 across the band; however, it works fine, hears well, gets out as well as it hears and I can copy the W1AW code practice on 80m CW as soon as the sun sets (3000 miles away).  It just doesn't match well at all.  In my case, I'm feeding with 160' of RG214/U, with the feedline bisecting the enclosed angle of the inverted vee.  Tried with or without a current balun, doesn't matter.

    However, if I crank the tower down to about 40' so the legs of the antenna are &quot;droopy,&quot; it loads up much better, showing a nice, smooth dip at about 3750 kHz, where it dips to about 1.8:1 or so.

    Crank the tower back up, back to same problem as indicated above.

    I decided to forget about it.  My amp's pi-L can handle the load just fine, and so can the autotuner in my transceiver.  160' of RG214/U with a 5:1 SWR at 4 MHz doesn't have much loss, so I stopped making antenna adjustments.  Can still get through first or second call to DX, usually.

    I still don't know why my 80m inverted vee won't resonate with the tower cranked up!  Possibly it's an imbalance in the antenna-to-earth or antenna-to-surroundings that becomes worse when the legs are at 45 degrees (to the tower, and to ground) than it is when they're more horizontal.

    At this point, I don't give a darn!

    WB2WIK/6[/QUOTE]<span id='postcolor'>
    That's an interesting problem Steve.

    Do you have the antenna symmetrically installed as it moves away from the tower? The coax drops straight down?

    Are your guylines insulated in non-resonant lengths?

    It would be VERY interesting to learn why that is happening.

    73 Tom
     
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