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The strangest thing with SWR and my tower

Discussion in 'CB Antennas' started by darklife, Jun 25, 2020.

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  1. darklife

    darklife Modulating Madness

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    After I put up my new A-99 clone antenna on my 20' of tower and mounted directly on top the tower (no isolation) I have a decent SWR of around 1.15 in the middle of the CB band, and about 1.2 on ch1 and ch40.
    Though this is great I noticed something very odd..



    When my 6' ladder is propped up against the tower a foot above ground on my porch (tower is against porch) the SWR is even lower!
    With the ladder touching the tower my SWR is almost flat, barely nudges my meter so it has to be below 1.15, probably more like 1.05. The channel edges are at 1.15 now. If I remove the ladder and put it away then my SWR goes back up a little bit.

    This is very odd to me because this is at the bottom of the tower so no idea how it could interact with it. I do not notice high common mode current so coax placement does not effect the antenna, I have it taped up against the tower.

    So this begs the question... How the hell does a little 6' ladder up against a 20' tower make the SWR better?

    I did a little experiment and took a thick piece of wire and clamped it onto the tower at around 7' and tied it near the ground about 3' away. This mimicked exactly what the ladder did so I kept the wire attached since it's not a burden. I mean why not.. it seems to improve things even though it makes absolutely no sense to me how or why. I also found if I put this attached wire along the coax cable to the ground it had no effect, it only helped when it was a few feet out much like the legs of the ladder while propped up against the tower.

    Can anyone explain to me how this is even possible? It's like some crazy voodoo magic going on!
     

  2. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Is the tower grounded or floating?
     
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  3. bob85

    bob85 Supporting Member

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    its normal for no radial end-fed antennas to be effected by the electrical length of the mast & coax outer braid,

    whatever radiates from the white part MUST return on something, in the absense of radials that something is your mast & coax & everything connected to them,

    the ladder & wire change the electrical length & common mode impedance of the mast, that impedance is seen in parallel with the load ( your a99 )
    & that is what changes vswr when you change mast or coax length,

    the meter is looking up the coax at whatever is terminating the far end of the coax, in your case the a99 + mast + surface of coax outer braid, each with their own impedance & current flow,

    the meter sees all 3 impedances in parallel terminating the antenna end of the coax,

    its the reason people get wildly differing results from & opinion of the same antennas,

    just hooking up ( or disconnecting ) a ground rod to the wrong length mast or coax can mess a perfectly good install up & create an rfi monster, changing a jumper length can do the same thing, fix a cmc issue or cause one,

    i would go with whatever gave the best signals over low vswr, then adjust vswr,
    start with a longer wire & trim for best signal, then tweak a99 for lowest vswr,

    there are many ways to reduce cmc, including disconnecting a ground rod or tagging a bit of wire to a mast to change its electrical length when you have a bad mast or coax length.

    good luck.
     
  4. darklife

    darklife Modulating Madness

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    As of right now it's buried at the base around 2' in dirt, and clamped to a porch post and rail. Basically using the 20' of tower as a really sturdy pole, obviously not safe to climb hence my use of a ladder.

    Doubt that counts as a real earthed ground, but I plan on adding a ground rod in the near future before storms hit. Not sure how the ground rod might interact itself with the aerial but time will tell.
     
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  5. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Darklife, does your antenna have radials?

    If so, how are the radials installed, horizontal or slanted down?

    You tell us a metal ladder, close or touching the tower, effects the antenna. Then how did you determine what you reported about the lack of Common Mode Currents (CMC)?
     
    #5 Marconi, Jun 25, 2020
    Last edited: Jun 25, 2020
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  6. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    I would not let my tower touch the ground or the ground touch my tower. Dirt has a way of destroying most metals over time.

    There are other and better ways of grounding to be sure.

    Check here for starters: https://www.worldwidedx.com/threads/good-info-about-grounding.136572/
     
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  7. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Does not come any where close to being a ground. The reason I asked is that with the tower essentially floating in respect to ground anything done to change the capacity between it and ground will in all likelihood have an effect on the overall impedance of the entire system. It's like touching one half of a dipole essentially.
     
  8. secret squirrel

    secret squirrel Lustrous Potentate

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    My first base antenna I ever put up. I followed instructions and just did what the locals talking base ar night instructed. It was a Radio Shack 1/4 wave ground plan. Three radials are 15 feet in the air at feed point U-bolted to 15 feet of old cast iron 2 inch pipe. Mounted to a 4x4 fence post the iron pipe rested on rocks at the base. For a ground I connected #4 gauge bare copper wire to the ground post on the antenna base it ran straight down to the 8 foot ground rod at the base of the antenna. I am sure the mast and ground rod were resonant. There was no isolation. The results little static SWR about 1.5 to 1 across band. One night I get on to operate and there is a lot of static all channels. A lot more than normal noise. I checked SWR. I thought the antenna go damaged by wind. SWR now 2.5, up then down the up. When I checked the antenna the ground connection was wobble loose at the ground rod. Tightened everything up and back to the norm. I’m not a radio expert by any means. I am a paramedic. I had training as a computer repair tech. I joke nowadays that my final tech cert test was on DOS and Windows 3.11. When I set up the antenna I was concerned with electrical grounding. I only knew that any static should be dissipated by a direct earth ground. And I had lots of 30 foot pines I hoped would take a lightning hit first. I used the #4 copper wire, overkill because I hoped the bare copper might help with static. I newer you can get a current on the outside of coax that causes static just like the old 8 inch brass hard drives would have ferrite beads on the data cables to prevent interference when running.
     
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  9. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    mfj line isolator or wind an rf choke 5 1/4 turns of coax 4" inside diameter and place at the feed point. should stop the CMC coming down the coax and give you an actual reading of VSWR.
    Much information on grounding here on the forum and online. DC grounding and RF grounding.
    I am sure the neighbors will appreciate it once you get the choke or line isolator installed, at 20 feet in height you have the potential to be radiating into power lines, computer speakers, household wiring, etc, etc
     
  10. darklife

    darklife Modulating Madness

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    No radials. I determined there wasn't CMC strong enough to mess with things by playing with coax length and position and none of it mattered. Also took a field strength meter and wanded it near the tower and noticed little to no radiation compared to the antenna itself which obviously threw off considerable RF as it should!
     
    #10 darklife, Jun 26, 2020
    Last edited: Jun 26, 2020
  11. darklife

    darklife Modulating Madness

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    That makes a lot of sense to me. I definitly will ground my tower properly soon. Like said this was a temp setup until I get it more permanent when money allows for ground rods and all.
     
  12. darklife

    darklife Modulating Madness

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    This is good advice and I eventually plan to try a current choke. Calculations I saw said 5 turns 4.5" but I'm sure it's not too specific, and placement should probably be a foot or two down from the antenna so that it has enough return juice to light up the antenna as an EFHW from what I read on ham forums.

    Thing is at first I did try a choke and it didn't seem to have any effect as I didn't have RFI to begin with near field, but that was before messing with tuning rings and length of the antenna. My antenna being a clone of the A-99 actually has an adjustable whip at the top element allowing quick adjustment without clipping a stupid wire, so I had plenty of play and fun time taking the bitch down and putting it back up a ton of times to get it right. It's a Tram 1498 but basically the same as a A-99 in all fiberglass the same, just built way better and cheaper if you can find one.

    Results so far with the antenna are far better than my homemade EFHW built with a T106-6 transformer setup as a fuchs style end fed, so something must be working well :D

    After the other comments though it's come to my attention that the tower itself is playing a role in the setup, so maybe that ladder was doing some voodoo magic to make it work better. Time to get proper ground rods I guess.
     
    #12 darklife, Jun 26, 2020
    Last edited: Jun 26, 2020
  13. bob85

    bob85 Supporting Member

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    Does it work better or is it just lowering vswr?

    IF you have a low mast impedance with high cmc you won't notice cmc on the coax as much as you may do if you properly isolate the a99 from the mast,

    My rule of thumb on mast length vs common mode impedance,

    floating 1/2wave multiple masts = high impedance low cmc,

    grounded 1/2wave multiple masts = low impedance high cmc,

    floating odd 1/4wave multiple masts = low impedance high cmc,

    grounded odd 1/4wave multiple masts = high impedance low cmc,

    you can see how willy nilly grounding without thought for cmc can increase cmc & spoil your radiation pattern depending on mast length,

    20 foot of mast burried 2ft in soil is 18ft of grounded mast & getting towards an electrical 1/2wave grounded,

    you could add a pole between mast & antenna to make it a grounded 3/4wave mast,

    plus it raises your current maxima higher above ground & obstructions,
    which is probably THE most important aspect of a vertical antenna when comparing signals.
    that and not having your coax & mast radiating accounts for practically all the difference people see between antennas in comparison tests,

    im sure an old timer called Marconi sent me down the common mode rabbit hole.
     
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  14. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Thanks for the added information on your setup. Over the years I've had several A99's and one acted really bad, but I thought the rest worked great.

    I could sometimes detect another CB vertical antenna making a better signal, but I don't think I ever did miss a contact that other nearby locals around me...were easily working.

    Over time I've had several other CB vertical high gain type antennas, and most of the time I didn't see much of a difference.

    IMO, most operators tend to put the EFHW antenna in a class of "deplorables."

    Keep us posted on what all you did to get your A99 clone to working right.
     
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  15. M0GVZ

    M0GVZ Sr. Member

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    Or just install the radials that should have been there in the first place. The Antron A99 has a radial kit that'll mitigate almost all problems especially when combined with a decent RF choke at the antenna feedpoint - 12 turns of RG58 round a FT240-61 toriod will choke off pretty much everything on 11m.
     
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