[Brian Howie]

I'm building a near copy of the F9FT 55 element. The original F9FT
antenna mounted the elements on tall plastic pillars, presumably to
minimise boom effects.

I have used use lower profile water pipe clips, suitably drilled ( the
boom is 15mm stainless steel water pipe) so the elements are closer to
the boom than the original.
Tall pipe fixings are available but I didn't like the mechanical aspects
of this approach.

When I modelled the antenna in NEC2 with the closer boom, I found no
difference to the impedance and radiation pattern .I had expected to
have to re-optimise at least the feed point. I got the same answer in
MMANA.

The questions I have a- Were Tonna being overcautious ? Is my
modelling software inadequate for boom effect modelling .

I think the F9FT design is based on the DL6WU design, so is possibly
less critical to these effects.


Brian GM4DIJ


--
Brian Howie

[bilou]

"Brian Howie" wrote in message
...
The questions I have a- Were Tonna being overcautious ? Is my modelling
software inadequate for boom effect modelling .
Hi
I think he used the same pilars on his 10cm beam that is fed from a
rectangular wave guide above the boom.
It seems he was also using them for 900Mhz yagis made for the american
market.
I can't be sure as I only have the 10 cm beam here.
It's elements are 4 cm above the boom.
One problem I remember from the 55 elements was QSB during high winds
but I am not sure if it was due to boom bending or rotor slackness.

[Brian Howie]

In message , bilou
writes

"Brian Howie" wrote in message
...
The questions I have a- Were Tonna being overcautious ? Is my modelling
software inadequate for boom effect modelling .
Hi
I think he used the same pilars on his 10cm beam that is fed from a
rectangular wave guide above the boom.
It seems he was also using them for 900Mhz yagis made for the american
market.
I can't be sure as I only have the 10 cm beam here.
It's elements are 4 cm above the boom.
One problem I remember from the 55 elements was QSB during high winds
but I am not sure if it was due to boom bending or rotor slackness.



Thanks. I've been reading around. It seems NEC models current flowing in
the centre of a wire, so simulating the boom with a thick wire isn't
going to work. I tried putting 3 thin wires spaced 1/4" about 1cm below
the elements, and it screws the impedance and radiation pattern.

I could simulate the boom with a wire frame model , but it might be
easier using taller pipe clips.

73 Brian GM4DIJ
--
Brian Howie