strange VSWR issue with antenna height

[towerdog]

I think I got it figured out just want some input on this.

Shakespeare 318 antenna, pulled up a tree with rope about 25 feet off the ground.
I set coil lead length and extended the top section for 11m operation per shakespear's manual.
It is fed with about 60 feet of double shielded LMR-240

I checked everything out with the antenna about 8 foot off the ground, VSWR=1.1 @ 27.400
I then pulled it up to the top, about 25 feet off the ground and this happens
VSWR=1.4 @ 28.100 and about 1.8 @ 27.400
I lowered it back down to about 10'
SWR and center freq go back down.

I am thinking one of two things.

(1) This antenna, although it does not specify a ground plane, still needs somewhat of a ground, so instead part of the coax screws with the impedance.

Strange thing is I had the antenna in a different part of the tree at about 20 feet up before and had no issue however the coax had more belly to it, basically ran along the ground, where now it goes free space at about 45 degree angle to the ant.

and/or

(2) My improvised PL259 connector at the antenna may have broken.
When testing today I noticed the radio end was not solid and I ended up just putting a N connector that's made for LMR240 on it and a PL259 adapter

could it be something else I am missing?

 

[Shockwave]

If a coax choke does not work, another trick you may be able to use is to tune the antenna off frequency by the amount it changes when you put it up. The resonant point moves up 700 KHz. when you raise the antenna. Tune it at 26.7 MHz. and it should be close to 27.4 when you place it back in the tree.

 

[Captain Kilowatt]

Agreed with shockwave. Any time you tune an antenna near he ground and then raise it the SWR minima point is likely to move as the antenna surroundings have changed. tune the antenna slightly in the opposite direction that the end result is and one should cancel out the other.

 

[towerdog]

according to the data sheet, for 10 meter ops the top section would be about 8" shorter, and the lead out of coil about 3/4" longer. I dont think it would be off this much just by raising it, but i will try what you said when all else fails.

 

[Robb]

according to the data sheet, for 10 meter ops the top section would be about 8" shorter, and the lead out of coil about 3/4" longer. I dont think it would be off this much just by raising it, but i will try what you said when all else fails.

Don't think I would trim/cut the antenna at all. You can't take it back. I would re-consider how/where I was mounting/installing it - though . . .

 

[towerdog]

yeah more i think about it, i think the absence of a metal mast is screwing it up, think i am going to zip tie some steel cable to the rope to a grounding ring and ground the antenna with it