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


???


thanks

On Sun, 09 Apr 2006 13:18:18 GMT, ml wrote:

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

The metal mast affects the performance to some degree, but I don't
know of a rule of thumb for predicting it. So the answer is, it
depends.

A fiberglass or other non-metallic mast will help, it you also use a
non-metallic coax line.





???


thanks

"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?)

In article ,
Wes Stewart wrote:

On Sun, 09 Apr 2006 13:18:18 GMT, ml wrote:

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

The metal mast affects the performance to some degree, but I don't
know of a rule of thumb for predicting it. So the answer is, it
depends.

A fiberglass or other non-metallic mast will help, it you also use a
non-metallic coax line.





???


thanks

thanks for the interesting reply's , i was originally wondering how it
would negatively effect it, which is my concern and knowing in what ways,

but the j pole thing got me sorta thinkn' once i know the downside
specifics' perhaps i might be able to turn it into a good thing??

cool

What I see in a lot of commercial installations is a Yagi with a boom
that's longer than it needs to be for the antenna itself, sticking out
the "back" past the reflector. That part attaches to the mast. If you
had a very long boom, you might want to counterbalance it with
something if you did it that way, and you might want some way to
support the end of the boom, like a non-conductive line up to the top
of the mast. The feedline in an installation like that would go back
along the boom to the mast, and then down.

Cheers,
Tom

ml wrote in :

In article ,
Wes Stewart wrote:

On Sun, 09 Apr 2006 13:18:18 GMT, ml wrote:

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

The metal mast affects the performance to some degree, but I don't
know of a rule of thumb for predicting it. So the answer is, it
depends.

A fiberglass or other non-metallic mast will help, it you also use a
non-metallic coax line.





???


thanks

thanks for the interesting reply's , i was originally wondering how it
would negatively effect it, which is my concern and knowing in what
ways,

but the j pole thing got me sorta thinkn' once i know the downside
specifics' perhaps i might be able to turn it into a good thing??

If you use a J-pole as the driven element of a yagi, it will not distort
the pattern much at all. You can fasten the J-pole to the yagi boom 1/4
wave from the top of the J-pole and still excite it from the stub.

But being in a yagi will change the impedance of the J-pole so you will
have to match it IN the yagi.

Another solution is to stack two smaller yagis side by side away from the
mast.

There will still be distortion but these techniques can lessen it
considerably.

--
Dave Oldridge+
ICQ 1800667
VA7CZ