SWR on 40 metre vertical

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by G7VQE, Nov 15, 2009.

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  1. G7VQE

    G7VQE Ham Member QRZ Page

    Following last Fridays weather rehearsal for the end of world or at least Dorsets version I've had to rebuild my fishing pole vertical......yes it went horizontal.:eek:

    Tempting fate I've made the new combination of pole and wooden support a little higher in the hope that I can somehow load the surplus to 40 section of fibreglass pole to use the antenna on 80 too,the support all told is now about 44 feet high so any suggestions there would be greatly appreciated.

    Back OT and I'm not overly impressed with the SWR on the new 40 metre vertical as according to my MFJ 949E atu the reflected power is about 5 watts shown with a maximum forward power a shade under 80 watts from the radio.

    I may be going paranoid here but I'm sure the SWR was better when starting with an initially long radiator prior to pruning,probably best at just over 6 MHz and then getting progressively worse as the radiator was brought to resonance at about 7.14 MHz.....during this tuning the radials (mixed lengths) were not adjusted at all.

    Would I be better starting off again with minimal radials tuned to 40 and tease out the additional wires that may or may not be holding climbers to the graden fence :eek: to at least give a level playing field or should I plan on some sort of matching network on what should be a resonant antenna.The radiator (1.5 mm2 insulated wire taped to the outside of the pole) does keep coming out shorter than calculation would indicate should be resonant too.

    I am researching this online and am presently suffering from too many articles showing some smiling amateur and his 1:1 SWR no tune $5 wire vertical that he just threw into a tree 5 minutes earlier and is now working VK on.

    If I sound despondent it's probably also where I'm gently weeping into the keyboard after having spent longer trying to tune the antenna than it did to build it,cheers in advance and if you reply Steve am I just seeing an impedance mismatch between 50 ohm coax and the feed point of this antenna and why wasn't it this far out prior to the original vertical blowing over?

    Thanks in anticipation,Dave.
     
  2. WD5ABC

    WD5ABC Ham Member QRZ Page

    That doesn't sound that bad, what is that, about 1.6 or 1.7 to 1? My 40m vertical is 30 ft tall made of conduit and I used 3 pieces of wire for top loading (I think about 37") to get it resonant. I use my rig's tuner to match it, you're not going to have that much loss at 40m. I think even at resonance my 40m vertical has about a 1.5:1 swr, it might be higher if I had better feedline.

    If you had a better SWR before you rebuilt it, maybe you didn't have as good of a counterpoise as you have now. A higher SWR is not necessarily always a bad thing!

    Good luck, hope you get it straightened out. Do you have a dip meter or antenna analyzer so you can see where the vertical is really resonant? An SWR meter doesn't really tell the whole story. If you don't have either of those, just throw down some radials and use the tuner. Or don't, most rigs should be able to handle an SWR of less than 2:1 without a problem.

    73,
    Kerry, WD5ABC
    Victoria, TX
     
  3. R3BU

    R3BU Ham Member QRZ Page

    The first thing to do is to provide at list two counterpoises with 0.25wl at working freq.Need to isert a Rig betwin them and see the rezonance.
    Atherwise at shassi(body) of your tuner may appear some unwanted HF remainings.
    The second- to difine at feedpoint of your vertical, what we may have there:a current side or voltidge side to be matched with coax.Most of the industrial tuners are nor capeable to match high impedance.To make them workable-we need to add just a coil.And we must bear in mind, that at the ends or tips of almost all antennas there is a high voltage and zero current.Make a drawing to see how the wave is lying in the body of your vertical.All is preety simple.May be it will need some additional reactance to compensate.
     
    Last edited: Nov 16, 2009
  4. G7VQE

    G7VQE Ham Member QRZ Page

    Thanks gentlemen,I may be a bit naive here but I must admit I was expecting a sweet spot or noticeable dip in the SWR curve where the antenna was resonant and there is but it's still fairly high as a ratio of forward to reflected power.

    Maybe I'm expecting too much but it would be good to see more RF leaving the radio as the atu indicates an output of 80 watts at best with output falling rapidly either side of the tuned point indicating a pretty narrow bandwidth,also having remeasured the radiator again this morning it's actually only about 26 feet long from the connection point which is way below calculated length.:confused:

    Maybe I should have built the antenna from alloy tube but given the cost of materials it probably would only be marginally more expensive to simply buy an MFJ 1792 and admit defeat......actually it's just too windy to raise and lower the antenna today so I'm just whining,I'll be back on the antenna probably Thursday when I'm next off work.
     
  5. R3BU

    R3BU Ham Member QRZ Page

    The MFJ-1792(1793) is so easy to bild at home.Even to improove it on 80m by making the capacity hat larger... On 40m and 20m it's a full size anyway...And as far as I remember at MFJ the price is not so far away if to by "row materials" for antenna in the nearest shop.
     
    Last edited: Nov 16, 2009
  6. G3TXQ

    G3TXQ Ham Member QRZ Page

    Dave,

    I'm slightly puzzled trying to follow what changed with the antenna/radials between the two different VSWR readings you think you got :)

    But a couple of observations anyway:

    * 80W Forward and 5W Reflected is a VSWR of 1.66:1 - not great, but not a problem

    * Wire taped to fibreglass does have a lower velocity factor than wire in the clear, so I would expect some "shortening effect".

    73,
    Steve G3TXQ
     
  7. G7VQE

    G7VQE Ham Member QRZ Page

    On the original version I had taped 4 bare 1.5 mm2 wires up the outside of the pole in an effort to fatten up the antenna,this time there is currently a single insulated 1.5 mm2 wire.

    I initially started this mark two vertical using some satellite coax running up the inside of the upper fishing pole section for a tidy look and connecting to the braid and centre core with both conductors connected to the 50 ohm feed coax centre.However after pruning away pretty well all the external satellite cable on the lower wooden support I would have had to resort to trimming the top end too so I gave up on that version and started again with the feed point at the pivot on the antenna and running up the outside.

    At this point I should mention there's a picture of the first version on my QRZ entry and that this next version simply has a longer wooden section at the bottom 12 ft long versus the 8 ft section shown,the feed point is currently where the antenna pivots.

    I couldn't play with the antenna today sadly as it's a bit delicate/flimsy and we're having a windy day again here:mad: however I shall be busting several blood vessels on it hopefully on Thursday......interestingly/frustratingly I could just hear YC6NE in Sumatra talking to PJ2/PE2B in Curacao at about 12:30 last night which is a bit of a tease.

    At the risk of badgering you further Steve what would be a typical SWR to expect as various information indicates that two sloping 1/4 wave radials should give a feedpoint impedance pretty close to the 50 ohms of my coax feedline?

    Yes,Alex at £200 here the MFJ 1792 is proving rather tempting,hi hi!

    Cheers,Dave.
     
    Last edited: Nov 16, 2009
  8. G3TXQ

    G3TXQ Ham Member QRZ Page

    Dave,

    You wouldn't expect tilted radials to produce the "classic" 50 ohm impedance unless they were angled at about 45 degrees from vertical. By the look of your arrangement they must be more like horizontal.

    The other point to mention is that, unless you have a good common-mode choke at the feedpoint, the coax braid is going to form a third radial which will likely change the feedpoint impedance from what you might expect.

    73,
    Steve G3TXQ
     
  9. G7VQE

    G7VQE Ham Member QRZ Page

    Thank you for mentioning the other point Steve....I couldn't wait until Thursday and as it had stopped blowing and was dry at last why let a little thing like darkness stop me sorting out the vertical!

    By re-routing the coax feed I managed to salvage about 15 feet of RG213 which I have now formed into an air cored pvc taped up "ugly balun" at the feed point.

    What a difference! the SWR is now as it should be with a distinct rise at each end of 40 metres but now indicating 100 watts forward power and as an added bonus I've lost the break through on my TV surround sound system which pleases the wife too.:eek:

    Again many thanks Steve,the daft thing is I wound a similar but smaller choke (less turns) for the 17 metre Moxon I built last year but didn't make the mental connection to take the same precaution when feeding the 1/4 wave vertical.

    Cheers,Dave.
     
  10. G3TXQ

    G3TXQ Ham Member QRZ Page

    Dave,

    Glad you've got a result!

    I find it often helps to picture coax as adding another radiating wire to the structure. So if you have a coax-fed dipole it's actually a "tri-pole" etc etc.

    Now go tell your fellow hams how you needed a 1:1 current balun at the base of your vertical and see what strange looks you get ;)

    73,
    Steve G3TXQ
     
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