RadioBanter - View Single Post

November 9th 04, 04:49 AM

In article ,
(Scott Mayo) wrote:

I'm thinking of dusting off my shortwave receiver and putting an
antenna in my attic. I have about 29' of horisontal space to play
with (in the most convenient direction, which runs more or less
NE-SW, in Massachusetts), or about 40' if I want to put more work
into it. Verically I have about 8', more in some places. I'm
considering a T2FD antenna, as I expect to listen across a lot of
bands and I don't have lots of space. Mostly I'm after BBC (which I
guess is now a distance station, mutter), time signals, and whatever
news broadcasts and music aren't All American All The Time. No real
interest in DXing. No interest in transmitting.

Since I don't want to spend the next month tacking up and taking down
the antenna, I'd kind of like to get it more or less right the first
time. So I have some questions about T2FD's.

1. My receiver is a Sangean portable (I think the 800a), with an odd
(RCA-style) external antenna jack. I understand that the antenna
impedience should match the receiver's. Will 300 ohm TV wire soldered
to an RCA jack work? Should I be looking at 75 or 50 ohm cable? (The
300 ohm TV cable has the advantage of being already in the wall).

The 300 ohm cable will work but 50 or 75 ohm coax would be better since
the antenna impedance of a loop antenna, not bothering to make a proper
T2FD is around the 50 to 100 ohms region. I don't know but expect the
radio's input impedance to be around that also.

2. T2FDs apparently like to be tilted (hence the name) about 20-40
degrees. I'll be lucky to get a 15 degree angle in my attic, and less
if I go out to 40'. How critical is the angle? What happens if the
wires are basically horisontal?

I would not worry about the tilt. Tilting it will allow you to make it a
little bigger in the same space but is not important. I would not bother
with the resistor either, just make it a wire loop.

3. With 29' of space to use, what frequencies am I just not going to
get?

The bigger the loop the lower the frequency it will work on. It is the
length of the loop that counts. Lets say you can make it 10 ' high and
29 ' long.

78 ' = 29' + 29' +10' +10'

This would be resonant around 12 MHz so it would work well on on the 25
meter band and up. Probably OK on 31 meter band and then worse as you
went lower.

The loop formula is length in feet (circumference) = 1005 / F MHz

If you need a lower frequency reception then you can use more than one
turn. Separate the turns at least a few inches.

4. Am I better off with a 40' (total length of both wires) dipole
instead of the extra fuss of the T2FD? Or do I really need to get a
long wire outside to get anything (which I don't want to do, as even
insulated copper seems to rot quickly if exposed to rain around
here.)

Depends on the receiver. The dipole will work but the loop will probably
work better.

Well, you have a portable that can overload on strong signals and the
loop will probably put out more signal than the dipole. You will most
likely do OK with the 300 ohm twin lead to a dipole.

If noise becomes a problem the loop will be quieter.

5. If I mostly want Europe, should I do the extra fuss needed to
fight for a N-S orientation of my wire?

For a one wavelength or more loop run the wire north / south for east /
west reception.

Thanks for any help. Email appreciated but I'll be looking here as
well.

I only post to Usenet and don't email replies. Good luck.

--
Telamon
Ventura, California