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40 M and I think mine's smaller than it should be...

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by G7VQE, Dec 17, 2009.

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  1. G7VQE

    G7VQE Ham Member QRZ Page

    I'm not sure if size really matters but I've a sneaking suspicion that as usual it does.

    The problem if it is one is that my home brew 40 metre vertical is nowhere near 32 feet high,at least the radiator isn't and currently stands at not even 30 feet albeit the feed point is actually 6 feet off the ground with the radials fanning out along an adjacent fence and down the support structure to the ground where they fan out.

    The SWR is excellent thanks to Steve TXQ pointing out that a choke wouldn't hurt at the feed point to stop the coax outer braid adding to the mix however this evening I sat reading Butternuts enlightening facts about radials and their take on SWR being misleading when assuming all's well with an antenna and masking poor efficiency.

    As I see it now my antenna is actually a bit short and not as a result of being a fat conductor for instance as the radiator is a length of satellite coax fed up the inside of a telescopic fibre glass fishing pole, so am I right to feel inadequate, could the antenna radiate more efficiently with it's missing wire added even though the possibly misleading SWR will then go out of whack or should I stop typing and let some kind soul (Steve) put me out of their misery?

    BTW previous attempts at building this antenna (it's the one shown in my QRZ entry) to calculated dimensions always seem to make it resonant at about 6.5 MHz also useable bandwidth in its present abbreviated form gives 1.5:1 at 7.00 MHz to near enough 1:1 at 7.2MHz so as a no code ham I'm happy with the bandwidth but just wondering if I'm missing an extra 10 dBs anywhere?:D

    Dave
     
  2. K8ERV

    K8ERV QRZ Member QRZ Page

    I'm just waiting for someone to say it.

    TOM K8ERV Montrose Colo
     
  3. WD5ABC

    WD5ABC Ham Member QRZ Page

    Dave,

    You've got a couple of problems. The antenna is too short to be resonant on 40, and you have a lot of bandwidth which tells me you've got plenty of loss. The elevated radials should be good. It sounds like you might not be able to make it any longer due to the physical limitations but you could top load it. With 3 or 4 wires coming out from the top (you can slope them down and tie guy ropes to it) you can top load it to get it resonant on 40m. To get the right length on the top loading wires you can use an antenna modeling program or just trial and error since it looks like it's easy to take down.

    If you read this forum very often, you've heard this before - low SWR isn't always a good thing. If you have 100KHz on 40m between 1.1 and 1.2:1 you've got lots of loss somewhere. Try the top loading, more radials is better, and force-feeding coax with a high SWR is asking for loss.

    Good luck, a 1/4 wave vertical should work great on 40m.

    73,
    Kerry, WD5ABC
     
  4. WD5ABC

    WD5ABC Ham Member QRZ Page

    I forgot to ask, how exactly are you feeding that satellite coax? If you tied it together at the bottom, fine, but if not, all bets are off.

    73,
    Kerry
     
  5. NL7W

    NL7W Ham Member QRZ Page

    Don't go there... ;):D:cool:

     
  6. G3TXQ

    G3TXQ Ham Member QRZ Page

    Dave,

    Forgive me - I can't remember some of the details:

    * Is the vertical directly fed with coax? If so, what type, and how long is it?
    * Do I presume the VSWRs you are quoting are measured at the shack end of the coax?

    Steve G3TXQ
     
  7. G3TXQ

    G3TXQ Ham Member QRZ Page

    Dave,

    I've done some simple sums which may put your mind at rest.

    Firstly, the wire is insulated which means the velocity factor is lower, and there will be further electrical "lengthening" wherever it is in contact with the fibreglass tubing, so the vertical may not be as short (electrically) as you think.

    If we assume that, wherever the antenna is minimum VSWR, Rrad will be about 36 ohms; and if you are seeing a minimum VSWR of 1:1; it implies losses of 14 ohms. That means you are losing: log(36/50)=1.4dB - not enough to be miserable about!

    By the way, assuming 14 ohms as the loss figure, I get a VSWR of 1.5:1 at 7.0MHz if it is 1:1 at 7.2MHz - that seems to tie up pretty well with what you are measuring.

    Hope that makes you sleep better ;)

    Steve G3TXQ
     
  8. G7VQE

    G7VQE Ham Member QRZ Page

    So many replies and so little time to compose my reply as I'm taking the shack master Christmas shopping today!

    Yes let's not go "there" all I will say on that subject is it's really cold here;).

    Back on topic and top loading may well be a bit of a problem with an umbrella effect with wire and guys making the easy up easy down a lot more cumbersome if not impossible.I'm guessing lengthening the wire and as has been said force feeding an high SWR is not good either,how about lengthening the radiator and adding a shed load more radials but bearing in mind putting them in the neighbours garden is a non starter?

    I read various bits on the net last night as I was typing my initial post but got the overall impression that a quarter wave vertical was about the easiest antenna to construct let alone the only one suitable for a 30 feet by 35 feet back yard so please don't tell me to put back my cloud warmer 40 metre loop as the wife wasn't impressed with that one.

    Theoretically I should be looking at 35 ohms in a perfect world and then looking at a web of radials to give the 35 missing ohms of the other half of the dipole,without an analyser I presently have little idea of where I am but the excellent SWR and bandwidth is a clue that all is not neccessarily well I know.:(

    The satellite cable I'm using has the centre core and braid twisted together at either end to provide a fat conductor with both parts running at the same potential....I did wonder for a while if there might be some capacative effect induced but reasoned that what would this reference against?

    [​IMG]

    These might help to show the antenna in some of its glory

    [​IMG]

    [​IMG]

    I considered throwing salt water over the washing on the rotary dryer on a regular basis but as can be seen any additional radials will only fan out in the same area I've already covered.....there are about a dozen I guess and all at least a quarter wave long but twisted this way and that including being threaded between the blocks that make up the paved area at the rear of the house and they are bare copper....some of the radials are connected to the bare galvanised steel wire that covers most of the fence horizontally at 1 foot spacings to support climbing plants so there's actually a lot of negative half to this antenna but not text book stuff.

    Keep the replies coming as I love all the attention and more importantly I might learn something,Merry Christmas all,Dave.
     
  9. G7VQE

    G7VQE Ham Member QRZ Page

    Thankyou Steve,I took that long composing the post below that you replied in the meantime.:eek:

    I have to admit I was hoping it wasn't a mile out as after you suggested the choke at the feed point things improved however in general when I read that one simply splits a piece of coax and runs a 1/4 wave vertically and then a bunch of radials out at the base it misses out the point that the SWR doesn't tell the whole story and that it might be happy but the antenna isn't.

    I had better go now as the wife is way past patiently waiting to go out and it looks like we might be going to Weymouth now at 25 miles away rather than Bournemouth at 40 miles.....at this rate we'll walk down the town centre and I'll be right in the.....trouble,cheers chap,Dave.

    I've just thought,those 2 feet long sections between the wood and fibreglass pole are aluminium so may add some capacative effect,the lashings are rope and non conducting?
     
    Last edited: Dec 18, 2009
  10. G3TXQ

    G3TXQ Ham Member QRZ Page

    Dave,

    Look at it this way - a lossless ground system would give you a feedpoint resistance of 36 ohms at resonance, and that's a VSWR of 1.4:1 so that's the ideal "aiming point". A minimum VSWR of 1:1, together with the VSWR bandwidth you quoted, suggest you're just 1.4dB off that ideal.

    Looks like you're having it as cold as we are - I've just been snow-clearing - reminds me of Christmasses as they used to be :)

    Steve G3TXQ
     
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