Diamond SGM-507 Mobile vhf/uhf Antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by VK4GAP, Sep 11, 2019.

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  1. VK4GAP

    VK4GAP Ham Member QRZ Page

    Just ordered a new one as they be cheap enough atm , has anyone pulled one apart and if so should there be a capacitor there? , those top 3 coils are shorted also by a solder blob is that should be too , i'm guessing that's the reason the wire burnt in half or a capacitor exploded (i did not see anything fall out of the tube btw ) , the antenna worked well when it did for a month or so .

    It's shorter .73m compared to the 1m long one i also have and just fits in underground car parks the way i have it set up .

    I found it best to heat up the alum tube with a gas pencil torch to soften/break the glue , it would not budge without that heat , a handy hint ! .

    Supposed to be 70w capability , 40w was the max i used Paul

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    Last edited: Sep 11, 2019
  2. VK4GAP

    VK4GAP Ham Member QRZ Page

  3. VK4GAP

    VK4GAP Ham Member QRZ Page

    You'll notice this below , thought i'd take a look and learn how to be more gentle when the new one comes apart to look at the vaporized capacitor .

    If one looks down into the hole where the elements poke in one will see that one side has a blank end at the bottom , the other end is the end to come off 1st to de-solder the wire/coil end , slight heat on the ends to break the glue to unscrew (i did a bit too much heat) , the glue was a bit proud and interfered with removing the coil and end on the other side .

    Re- Coil , 1 turn each end soldered/shorted , on this one there was a solder splash going from 1 end nearly to the other across the top you can just see the remnants in the last picture (splash removed in picture) . Paul

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  4. VK4GAP

    VK4GAP Ham Member QRZ Page

    No longer a mystery , 4pf capacitor .

    Also this one is slightly different , now no grub-screw in for the top element , i did not bother to see if it was pressed in or screwed in .

    After throwing it back together then checked swr i found in this situation it was high after 147 and nice and low on 145 so snipped off 8mm of the top element (that top round thing just pulls off) and now flat swr from 145 to 148 , it did not seem to effect 70cm which i hardly use mobile anyway , 70cm is a bit high at 1.7.1 i maybe and might look into that using the other ones elements for testing purposes .

    I guess the 1st antenna could have been a Friday afternoon assembly job going by the solder splash's and the top 3 turns of the main coil soldered (this newer one is only the top 2) , all good and works well after doing some tests on far off repeaters and no more getting out of the vehicle! .

    ps > if one gets inside one of these make sure there is a large enough solder blob on the top of the bottom coill so the tilt over function makes good contact ....

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    Last edited: Sep 26, 2019
  5. VK4GAP

    VK4GAP Ham Member QRZ Page

    Ok , the last input on the Antenna .

    1st , it does work well and seems just as good as the higher gain 1m long mobile antenna imo after several more trials .

    It looks like that 4pf cap has to be 4pf as i have tried a 4.7pf 1KV and it shifted resonance up around 1MHz on the blown cap antenna on vhf .

    Also , both antennas get warm on high power , guessing 45c at the cylindrical tube using 50w vhf at a 3 minute time-frame using the timeout timer on the radio , 25w was only just slightly warm to the touch , i did not try uhf as it was getting too hot out there today in the Sun and gave up .
    ps- with any amount of high vswr i gather that heat developed will be more significant

    That's it from Him and cheers from Me , I never turned up probably out boozing with AM .

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