HD7698 shorted vhf side

[spudbudy]

I just assembled a HD7698p everything went together well however, upon doing a channel scan almost no channels came up which was way worse then my old antenna with 2 amps on it. After hooking up 1 amp at the tv I got improved signal strength but no more channels. uhf channels very poor signal strength. tried hooking up the 2nd amp near the antenna and the indicator light shows a short so it won't work. I took the coax box off the antenna and checked everything all looked fine. I checked the uhf side with an ohm meter and it showed no shorts to each other or to the boom. Upon checking the vhf side it shows shorting to each other and to the boom. I've inspected everything I can see and can't located the source of the short either to the boom or to each other. I'm assuming this is causing my poor reception problems. I believe I've got all the antenna arms in the correct position as the alternate top to bottom every other one. The antenna is on a ten foot mast and I'm about 38 miles from the towers zip 60021. any ideas on what I should be looking for and how to fix it my wife is unhappy she can't watch the channels we had before. this is why I replaced the antenna with this one.

[winegard]

Hans informed me he spoke to you over the phone. If you still need assistance please let us know.

Thanks!

[re_nelson]

I checked the uhf side with an ohm meter and it showed no shorts to each other or to the boom. Upon checking the vhf side it shows shorting to each other and to the boom. I've inspected everything I can see and can't located the source of the short either to the boom or to each other.

Amazing and quite coincidental! Just a few moments ago, I posted about this same situation in another forum:

http://www.digitalhome.ca/forum/showthread.php?t=142786

In my case, it's an HD7696P that I always felt was an under-performer on the VHF side relative to the published gain figures. In fact I even obtained a replacement balun cartridge in hopes of resolving VHF gain. Like spudbudy, I checked the lines with an ohmmeter.

The UHF lines were isolated from each other. But the VHF lines were shorted although there was no continuity to the boom. Although I could talk to Hans about this, perhaps an explanation of how spudbudy was directed to solve the problem would be appreciated.

[re_nelson]

The UHF lines were isolated from each other [on the HD7696P]. But the VHF lines were shorted although there was no continuity to the boom. Although I could talk to Hans about this, perhaps an explanation of how spudbudy was directed to solve the problem would be appreciated.

Hans Rabong of Winegard contacted me and explained that the shorted VHF phasing lines is normal and is by design:

The HD7696P antenna will read a DC short with an ohm meter between the two sides of the VHF phasing lines because there is a 3" rivet holding the reflector elements and plastics to the boom. The way the antenna is tuned the RF signal is not affected by this DC short.

[re_nelson]

Let me followup on this issue with more information.

After consulting with the extremely helpful Hans Rabong at Winegard, a replacement HD7696P antenna was sent for an A/B comparison.

I carefully examined both the original HD7696P and the the replacement and couldn't discern any obvious physical difference between the two.

But when connected to a signal analyzer, the difference was remarkable! The UHF portion of both was within a 0.5 dB -- so no change there. But the VHF performance was anywhere from 5.5 dB to 7.8 dB improved on the replacement HD7696P. The same balun cartridge was used on both the original and replacement antennas to eliminate that factor as a variable.

Although I've yet to find the underlying cause, suffice it to say that the HD7696P (based upon "real world" measurements of the replacement) does match the published gain figures on the VHF side.

[re_nelson]

Back on 01 AUG 2011, I wrote:

I carefully examined both the original HD7696P and the the replacement and couldn't discern any obvious physical difference between the two.

So much for my "careful examination". I couldn't see the forest because I focused on the trees. With the help of a friend, I now know why the the original didn't perform as well as the replacement that Hans Rabong sent!

The VHF elements on the original were assembled incorrectly. They are supposed to alternate on the top and bottom of the boom as shown in the "Engineering Specifications" diagram. I was so focused on measuring for shorts and checking the lengths of the elements that that this obvious difference wasn't even evident until pointed out to me when my friend looked it over.