Hi Richard

My gut feel is that adding elements to a set top antenna is a pointless
exercise. I think your biggest problems to obtaining a sharp low null
pattern wil be internal (room/building) reflections and detuning by
being close to other objects. (Element detuning will affect gain/nulls
and as soon a you walk near it or place it within a few metres of any
objects it will affect the pattern badly) The elements will also lower
the antenna input impedence. This may or may not be a problem to the
following amplifier.

I am suprised that with transmitters that far away (I guess 50 miles not
50 metres) you are even considering using a set top antenna. I assume
you have a good reason for this so there is no point embarking on
suggestions of an external one.

If I was limited to using an internal antenna I'd make a 3 element yagi
specifically for 187MHz and tune it for maximum front to to back. Given
the bandwidth of a TV signal though I doubt I'd reach a good figure. I
would follow this by a preamp also tuned to 187MHz. Even then I doubt
I'd get pleasing performance with it internally mounted.

The only other suggestion I can make is to go ahead and try it and
report back to the NG. Keep in kind that you have to do a good job of
tuning the elements you add (spacing and length) to get good nulls. You
may in fact want to try a single quad loop reflector and add a air space
tuning capacitor in the loop. The idea is to resonate it to about 5%
higher than the operating frequency so you'll need to make it slightly
larger than that again to allow for the capacitor.

I apologies for not being much help!

Cheers Bob VK2YQA

Richard Lane wrote:
Richard Lane wrote:

I live 50m from TV transmitters, want to enhance ch9 (187mHz) without
using a roof antenna and have a Terk TV-5 amplified set top antenna.
Is it possible to place directors ahead of its dipole and a reflector
behind to increase the forward gain and null some of the multipaths?
Dick

The TV-5 antenna is an angular figure 8 in the horizontal plane having
an overall span of 64cms but measuring around the fig.8 periphery is
86cms. In order to give ~ 8dB gain I was proposing to place 2 directors
in front of the fig.8 and a reflector behind with 0.15 wavelength
spacings and was seeking comments on the idea.
Dick