Hi again Jim

I'd suspect you need an attenuator of maybe 60dB in 6dB steps! I cant
see the 10dB variable one as being useful. Of course at 50dB or more
coax leakage might be an issue!

The other researcher you mentioned most likely dropped to signal
strength only mode when they got close. Antenna directivity up close in
my view isnt a viable way of doing it. As a rough guide you get a 6dB
change in signal power everytime you double or halve the distance. This
means that if you maximum signal measured distance is maybe 6ft and it
must work out to 2000 yards then you have to allow for 60dB of
measurement range. This is further complicated by the turtle antenna
being at ground level, in fact in the mud and dirt. I dont know what
that loss represents but I'd add 30dB to be sure. Your whole system then
must have some method of measuring over that range of 90 odd dB and have
useful directional nulls etc doing so. This is where the big step
attenuator and antenna changing/removal help.

Should the antenna work okay? I guess the question is did you include
the ground in the modelling? Obviously you'd get some major sky
direction lobes/response from ground reflection but the worry is how it
affects overall directivity. My gut feel is that the F/B would be
markedly different and nowhere near the 48dB. At 1M above ground there
may be sufficient detuning of elements to widen the frontal lobe but I
doubt it would be a huge excursion. Do you find the null is much sharper
than the frontal lobe? The bottom line though is that in your
application it isnt as critical as someone (like amateurs) trying for
maximum performance. This means I should stop complaining!

Okay on the screening. Well as I mentioned the test is to plug a 50 ohm
load in the antenna socket and see if it responds at a distance of say
5ft. If it doesnt then dont worry about screening. If it does it depends
on how much and then whether things like coax leakage have to be
factored in.

I'll stop ranting/waffling on now..!

Cheers Bob

Jim wrote:
True, I haven't thought of everything, but I have done this: