"ml" wrote in message
...
hi


i've noticed a few installations of say a 2m beam vertically mounted to
a regular metal tv style mast

from looking at it, you have the mast(metal) going between 2 elements
and longer than the elements

so i was wondering as this is a common mounting style and method

how bad is the beam pattern for rx and tx ? effected by the metal mast?



it also got me thinking i've noteced some beams as in the above use a
fiberglass boom while others use metal

how does this effect patterns and efficiency

both from a practical point of view and technically speaking

Here's a partial answer:

I had a 2M copper pipe j-pole suspended vertically and well away from any
other metal. I ran the coax through a switchable attenuator into a radio
tuned to a local repeater with fairly steady chatter. I set the attenuator
to put the radio deep into the noise -- a few dB away from Gone. Then , I
took a length of mast, held it vertical and slowly approached the j-pole. I
had to get within about one inch to affect the chatter. The direction from
which I approached didn't seem to matter. Next, I did a similar experiment
on transmit while watching reflected power and saw essentially the same
effect.

Theoretically, I think the added capacitance simply detunes the radiating
element when a mast is really close, meaning an inch or less. Otherwise,
the mast isn't seen as director, since the size is so far off. (Guessing is
allowed, right?)