I have a PC FM radio peripheral. It has a standard telescopic steel
aerial that plugs into a base. This works quite well.

However, I would like to recieve some signals that are too weak for
this arangement. I have an external TV yagi, but attaching it either
reduces the recieved signal or increases it only slightly.

Could impedence differences be an issue here? If so, what is the likely
impedence of the yagi and the peripheral?

On 15 May 2005 19:37:52 -0700, wrote:

I have a PC FM radio peripheral. It has a standard telescopic steel
aerial that plugs into a base. This works quite well.

However, I would like to recieve some signals that are too weak for
this arangement. I have an external TV yagi, but attaching it either
reduces the recieved signal or increases it only slightly.

Could impedence differences be an issue here? If so, what is the likely
impedence of the yagi and the peripheral?


Yes, chances are that the steel aerial is connected at 75 ohms. Your
yagi is most likely going to be frequency specific unless you are
talking about a log periodic and calling it a yagi.

The standard external TV antenna is a Log Periodic and requires a 300
ohm connection which these days is usually adapted to a 75 ohm coax
via a small balun.

While you may already know this, I will say for clarification to those
who may not...

The Yagi is a beam typically of similar length elements which will
almost look like a rectangle when all the outside tips are connected
by a line. The log periodic will almost look like a triangle in the
same way. Each element considerably smaller and may be pointed
forward to form 'V' shaped elements.


--
73 for now
Buck
N4PGW

" bravely wrote to "All" (15 May 05 19:37:52)
--- on the heady topic of "Could a Balun Be Useful Here?"

ma From:
ma Xref: aeinews rec.radio.amateur.antenna:30429

ma I have a PC FM radio peripheral. It has a standard telescopic steel
ma aerial that plugs into a base. This works quite well.

ma However, I would like to recieve some signals that are too weak for
ma this arangement. I have an external TV yagi, but attaching it either
ma reduces the recieved signal or increases it only slightly.

ma Could impedence differences be an issue here? If so, what is the
ma likely impedence of the yagi and the peripheral?


Hmmm, those are usually 300 ohms at the top of the mast. What comes
into the building, is it a 75 ohm coax or a 300 ohm balanced twin lead
tv cable? If it is 300 ohm there might be a noticable difference. Have
you tried a little can tv transformer? Might there be an FM trap on
the antenna system? Do you have any idea of the input Z of the radio?
It may be 50 or 150 ohms, who knows? If your antenna cable is a 75 ohm
coax you may need to adapt this to whatever input Z the radio has but
it shouldn't be as critical as the 300 ohm balanced line in my opinion.

A*s*i*m*o*v

.... Bad day: When the bird singing outside is a buzzard!

Buck wrote:

The Yagi is a beam typically of similar length elements which will
almost look like a rectangle when all the outside tips are connected
by a line. The log periodic will almost look like a triangle in the
same way. Each element considerably smaller and may be pointed
forward to form 'V' shaped elements.

Well then it would appear the antenna in question is probably a log
periodic then :-)

I will get myself a standard 75-300 ohm balun and see what I can do.

As for FM traps etc, no there's nothing like that. It's possible it's
not optimised for the FM band though, although it certainly is for
slightly higher VHF transmissions.