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Xptical 11-Sep-2013 12:47 AM

Folded Dipole Questions
Hi all,

I made a folded dipole antenna tuned for VHF CH#7. The first day I set it up, it had one wire on top and one on the bottom. It worked just fine.

I moved some stuff around and re-cabled my living room the next weekend. And with the antenna in the same spot and same orientation, I had no signal.

Tonight, I decided to move it around a bit. While I was moving it, it fell off the side of the shelf into a "top wire at the front ; bottom wire at back" orientation. Bam! I had signal.

On top of all this, I get the best signal with the long side of the antenna at about 90-degrees. The station is about 110 degrees. If I aim the antenna perpendicular to the station direction, I get no signal.

I'm planning on modding the antenna even further into a Yagi (or maybe stacked dipoles) to get the best possible signal from this station. But I need some help understanding what I'm seeing with the orientation.

Any tips?

WMUM-TV (real ch# 7) at 111 degrees magnetic.

GroundUrMast 15-Sep-2013 7:32 AM

If you connect directly to a coaxial cable, you introduce an imbalanced condition that would likely skew the radiation pattern due to the coax shield being coupled to the antenna element.

The typical folded dipole will have a higher impedance than a simple dipole. How are you matching the antenna to the feed-line? A 300Ω/75Ω balun/matching transformer would be an appropriate choice.

Also, indoors is rarely an ideal 'test range'. HVAC ducts in the wall and any number of other reflective items can alter the antenna pattern and tuning.

Have you modeled the antenna in 4NEC2 or another antenna simulation program?

Xptical 15-Sep-2013 5:36 PM

I'm using a radio shack BALUN off the dipole. That's feeding into a VHF/UHF coupler.

I have moved the antana to a window on the east corner of the house. Internal structure shouldn't be an issue.

I haven't modeled it. I got the freq of the station and ran it through the formula on this site:

I'm getting signal numbers from a Linux prog called zap:

Anyway, I'm getting good signal now. I just wondered about the individual dipole wired being front-and-back or top-and-bottom.

GroundUrMast 15-Sep-2013 7:20 PM

Thanks for the links, they help visualize the antenna element you're working with.

For commercial OTA TV and FM signals in North America, the folded dipole is oriented so that the uninterrupted element section is either above or below the section with the feed-point gap.

The Antennacraft Y5713 and Y10713 use the folded dipole as the driven element(s) in their design. Note that they opt to connect the center of the side opposite the feed-point to the metal boom... that is a minor variation the basic design you're using and does not change the function. Also, the feed point could be placed above with no effect on performance.

If the feed-point is forward of or in back of the continuous portion of the folded dipole element, the two halves will be slightly out of phase with each other (relative to the arriving signal). This would theoretically result in reduced performance. I would not expect the delta in performance to be of great significance though because the phase difference is not more than a few degrees.

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