Specific interference on the VHF Business High band

[videobruce]

Yes to what? Raises or lowers the impedance? Actually, is it changing the frequency and which way?

(Smith charts I don't even begin to understand, besides it's way beyond my pay grade.)

 

[prcguy]

Yes to what? Raises or lowers the impedance? Actually, is it changing the frequency and which way?

(Smith charts I don't even begin to understand, besides it's way beyond my pay grade.)

Changing the size of the U changes impedance within its useable range. You can make it too big or small and that may affect the resonant freq. I don’t know if making it bigger raises or lowers the impedance.

 

[videobruce]

I was referring to it's length, leaving the width at 1" per the original design for 146 MHz. The gap between the two driven element halfs is 1". Doesn't that fix the width of the 'U' leaving the only adjustment choice length??

 

[prcguy]

I was referring to it's length, leaving the width at 1" per the original design for 146 MHz. The gap between the two driven element halfs is 1". Doesn't that fix the width of the 'U' leaving the only adjustment choice length??

Yes

 

[videobruce]

It was asked if I noticed changes in the pattern of the interference with any weather changes. Up to a couple of weeks or so ago, there wasn't since there weren't any real changes in the weather,
but,
from last weekend to now the interference has completely stopped, or at least drifted outside of the frequency area of interest. Nothing as far as I can tell. The temperature has dropped 20+ degrees overall 4 or 5 days ago and we have had some, but not much rain. Before, we were in a dry spell and in the upper 80's during the day. Now it's mid fifties and mid 40's at night.

That can't just be coincidence. Any thoughts on this??

 

[videobruce]

Attached are scans of the corrected yagi with shortened leads from the feed, a trimmed director length (less than 1/4" was trimmed off and remounted driven elements (my 1st cut was 1/2" which was too much, I cut two new pieces). I also spaced those further apart on the 'cross' fitting per the original design (see pics).

I tested on a rear porch securing to to a wooden railing, later having my XYL hold the antenna in different positions away from the porch roof as much as possible while I watched the SA to see if the notch varied.

This behaved differently as to the level settings that I'm use to using. I included scans w/o a load (after normalizing w/ a load), another scan with the load, and two scans w/ the antenna attached (via a 15-18' RG-58 cable), the last pic was w/ the marker table showing.

 

[Ubbe]

I would regard that dip to be the S11 value and any single band antenna I have measured are around that same 20dB value indicating that the antenna has a low SWR. If it's only a 15dB drop or less then something isn't right.

If you cut elements too short then just move them out so that their tips have the correct distance. It doesn't matter much if there's a bigger gap in the middle, especially if you splice the coax a bit more that will then replace the missing elements.

/Ubbe

 

[videobruce]

I showed those two scans, one with a 50 ohm load connected and the other with no load since the overall initial settings to bring the trace on screen was considerably different than most other TG scans I have done. I normally set the 'Norm Ref Pos' to 90%, here I had to reduce it to 50%