[TowerTalk] Comet GP-6NC question

Jeff DePolo jd0 at broadsci.com
Sun Dec 6 15:53:08 EST 2020

> I obtained a Comet GP-6NC vhf/uhf vertical from a silent key sale. The
> antenna is listed as 155-465 MHz.So my question is, has anyone modified this
> antenna for the ham bands? I'm looking for anyone with experience with this
> antenna before I try myself to modify it. Always good to know others hints
> and tricks to start with!

Without replacing the elements and passives, it will never work ideally on the ham bands.  If you leave the element lengths as-is, and just deal with improving the feedpoint match, the antenna will work, albeit with a reduction in gain and a bit of downtilt.  But if you want to play around to just try to get the match right, take some aluminum foil, fold it into a strip about 2" wide, and wrap it around the radome down the near the bottom above the radials, held in place with a rubber band.  Transmit with low power (like, a fraction of a watt) in the middle of the band of interest (e.g. 146 MHz or 445 MHz), watch the reflected power, and start sliding the foil collar up the antenna until you get a "dip" in the reflected power.  Once you find the spot where the dip occurs, then vary the width of the foil by unfolding it or folding it more until you get the match as good as you can.  Then, repeat the same thing again with another foil wrap up higher on the antenna.  

What you are doing is adding capacitive loading to electrically lengthen the element. 

Once you've empirically found the best locations and widths of the foil straps, mark the top and bottom of each foil collar with a sharpie and then remove them and replace them a with HVAC-style foil tape stuck to the radome, and then cover with a layer of good electrical tape (Scotch 33+ or 88) for weatherproofing.

Whether or not you'll be able to get the dips on 146 and 445 MHz to be coincident will be a crap shoot, but maybe you can get something workable on at least one of the bands.

					--- Jeff WN3A

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