Bob Bob wrote:
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
Today I made up a 2 director, 1 reflector "beam" out of 1/4" dowels
wrapped with cooking foil, the reflector was planned to be 26cm to the
center of Terk dipole, Terk to 1st director is 16cm as is 1st to 2nd
director. The reflector is 80cm, 1st director is 73.6cm, 2nd director is
71cm. It appears to make a big improvement, however it "works best"
positioned with the Terk about 2cm behind the 1st director! I have no
field strength instruments but though "snowy" the sound is quite solid.
I have not yet tried moving the whole assembly around the condo living
room though given the Terk amplifier I should be able to use several
meters of coax before the loss starts to show in noise figure.
The "Association" does not allow external antennas and as I watch very
little TV I resent paying $50/m for the one or two hours I watch each
month. I am a retired microwave device engineer but not an antenna expert.
Thank you for your interest in my experiment.
Dick