I like to put up a big hi gain FM broadcast band antenna (by big I mean 15 -
20 feet).

I can get the formulas for the spacing of the reflectors and directors from
literature, but it always says further optimization will be necessary.

This 'further optimization' might prove difficult and I wonder if anybody
here has experience with this or has built one of these antennas??

Uwe

UWE wrote:
"I would like to put up a big hi gain FM broadcast band antenna."

1.5 wavelengths is about 15 feet near the center of the FM broadcast
band. Kraus gives an example of this 6-element Yagi. It has a radiator,
reflector, and 4 directors in Fig. 8-29 on page 248 of his 3rd edition
of "Antennas". Directivity is about 12 dBi and bandwidth is 10%.
Construction is not too critical because the space between its elements
is fairly wide.

I have built Yagis and been pleased with their performance.

Best regards, Richard Harrison, KB5WZI

Hi Uwe

I'd suspect that the optimisation you are talking about is not going to
make a lot of difference to your end use. You tend to be able to tune
yagi's for best forward gain or best front to back ratio but not both
together. You have the added complication that if you want to cover the
entire FM band a single yagi would be a major compromise and it might be
better looking into a different kind of design like a log periodic or
axial mode helix.

As a general rule too, the longer the yagi the narrower bandwidth its
operation is. My gut feel for a 20ft FM band yagi antenna would be only
for use over maybe 200kHz. It would of course still work further from
the design frequency but at reduced performance.

As an aside some of the optimisation will be due to slight material
differences and deviations from original mounting techniques (eg through
boom vs on top, proximity to buildings etc)

Would you care to share you reasons for the high gain design? Is it one
distant station you want or just for general DX use? Might you be trying
to reduce interference effects?

Cheers Bob VK2YQA



Uwe wrote:
I like to put up a big hi gain FM broadcast band antenna (by big I mean 15 -
20 feet).

I can get the formulas for the spacing of the reflectors and directors from
literature, but it always says further optimization will be necessary.

This 'further optimization' might prove difficult and I wonder if anybody
here has experience with this or has built one of these antennas??

Uwe

Bob, the reason to want the high gain is as follows.

I live in Maine south of the Canadian border, close but not quite close
enough to receive Canadian FM stations. I can receive one station faintly
using a Radio Shack BC band antenna and wanted to expand on that with a
better antenna and possibly a change in IF filters in the tuner I use.

And I thought that a very high gain antenna with its narrow pattern would
also aide in adjacent channel rejection. I realize that it could be a
problem covering the entire BC band and I am not set on any particular
design, I first wanted to see what people here had to say.

Regards Uwe
KB1JOW

From: Bob Bob
Newsgroups: rec.radio.amateur.antenna
Date: Tue, 16 Nov 2004 19:18:13 +1100
Subject: Homebrew high gain FM broadcast antenna ??

Hi Uwe

I'd suspect that the optimisation you are talking about is not going to
make a lot of difference to your end use. You tend to be able to tune
yagi's for best forward gain or best front to back ratio but not both
together. You have the added complication that if you want to cover the
entire FM band a single yagi would be a major compromise and it might be
better looking into a different kind of design like a log periodic or
axial mode helix.

As a general rule too, the longer the yagi the narrower bandwidth its
operation is. My gut feel for a 20ft FM band yagi antenna would be only
for use over maybe 200kHz. It would of course still work further from
the design frequency but at reduced performance.

As an aside some of the optimisation will be due to slight material
differences and deviations from original mounting techniques (eg through
boom vs on top, proximity to buildings etc)

Would you care to share you reasons for the high gain design? Is it one
distant station you want or just for general DX use? Might you be trying
to reduce interference effects?

Cheers Bob VK2YQA



Uwe wrote:
I like to put up a big hi gain FM broadcast band antenna (by big I mean 15 -
20 feet).

I can get the formulas for the spacing of the reflectors and directors from
literature, but it always says further optimization will be necessary.

This 'further optimization' might prove difficult and I wonder if anybody
here has experience with this or has built one of these antennas??

Uwe

Ewe wrote:
"I realize that it could be a problem covering the entire BC band and I
am not set on any particular design."

Kraus` 6-element Yagi on page 248 of the 3rd edition of "Antennas" has
12 dBi doirectivity and a 10% bandwidth. FM channels are 200 KHz wide.
10% of 100 MHz is 10 MHz. That is 50x what you need for a single
channel.

If you want to cover the entire FM band with an antenna, you can do it
easily with a rhombic. Arnold B. Bailey`s rhombic in "TV and Other
Receiving Antennas" scales to: two #10 wires 72 feet long, separated by
36 feet at the midpoint. Overall length is 61 feet. Surge resistance is
600 ohms, and 600 ohms is used as the terminating resistor value. Bailey
uses 300-ohm twinlead as a transmission line. Gain is 14.5 dBd (or
less). Bandwidth is 30% for 1 dB down. That`s 1.5x the width (20 MHz) of
the U.S. FM band.

What you probably need is height. Signal drops fast in the shadow of the
horizon or an obstruction.

Best regards, Richard Harrison, KB5WZI

Hi Uwe

I cant resist... Brrrr COLD! I visited Toronto early this year and
havent gotten over the throat irritation yet! It was about 33C when I
left Sydney and -25C when I got to Toronto!

Well then I can now see the use of employing the narrowest band design
possible, assuming you only want the Canadian station. Personally I
wouldnt want to limit its bandwidth because I *might* one day want to
use it for another distant station.

Is it worthwhile doing an analysis of those stations that might
interfere? Like if the interfering source is at 90 degrees heading from
the station you want, anything that starts as a dipole element will
help. If its directly behind you may want to adjust for best front to
back, even to the point of tuning the reflector with some C. This BTW is
much easier on a quad reflector than a yagi. (ie make a "Qagi")

My theoru isnt good on this, but I can see you want a design that has
got low sidelobes and good front to back. Yagis can be optimised for all
manner of things so one design you see may work great for f/b but bad in
sidelobes. Is a phased array better? (ie mount your boom at right angles
to the station and use 2 element designs) How does a quad or helix
compare? Does choosing circular polarisation help? This is actually a
more difficult problem than first thought.

My path would be not to think too much, but put up a helix and see how
it performs. I use to have one on 2M for satellite work that I also used
for terrestrial TV and FM. (Very wide band) I was however in a zero
interference area. Given though that I guess you want to go with a yagi
design, my thought is to build it to the Canadian station frequency with
a tuned refelector and also see what happens. When you have built it
plot the pattern at various frequencies. Have a look at Mr Cebiks web
page on OWA yagi's. (http://www.cebik.com/2mowa1.html) He often shows
radiation patterns and lots of other data. Well worth a look.

Apologies for the length!

Cheers Bob VK2YQA





Uwe wrote:
Bob, the reason to want the high gain is as follows.

I live in Maine south of the Canadian border, close but not quite close
enough to receive Canadian FM stations. I can receive one station faintly
using a Radio Shack BC band antenna and wanted to expand on that with a
better antenna and possibly a change in IF filters in the tuner I use.

And I thought that a very high gain antenna with its narrow pattern would
also aide in adjacent channel rejection. I realize that it could be a
problem covering the entire BC band and I am not set on any particular
design, I first wanted to see what people here had to say.

Regards Uwe
KB1JOW

On Mon, 15 Nov 2004 23:53:47 GMT, Uwe wrote:

I like to put up a big hi gain FM broadcast band antenna (by big I mean 15 -
20 feet).

I can get the formulas for the spacing of the reflectors and directors from
literature, but it always says further optimization will be necessary.

Uwe,

Check out www.cebik.com he has an article "some notes on FM BC
antennas". Or at least something like that. He does extensive
antenna modeling.

One of the problwms with FB broadcast antennas is that 88-108 (US FM
allocation) is a very wide band percentage wise to make a good yagi
for. Programs like YO, QY4, are likely inadaquate for wide
bandwidth antennas so you may end uo with EzNEC or one of the
NEC varient calculation programs for antenna design.

If you were to take a 13b2 and lengthen the elements they would be too
closely space to get the same performace as the origional. You have
to scale length, spacing (and boom length) and diameter by the same
factor to get the same antenna at a different frequency. IE:
everything would be about 1.45 larger to optimize for 100Mhz.

Good luck.
Allison

"Uwe" wrote in message
...
I like to put up a big hi gain FM broadcast band antenna (by big I mean
15 -
20 feet).

I can get the formulas for the spacing of the reflectors and directors
from
literature, but it always says further optimization will be necessary.

This 'further optimization' might prove difficult and I wonder if anybody
here has experience with this or has built one of these antennas??

Uwe

Uwe,

I suspect you can buy a commercially made FM antenna for the cost of
materials that it would cost to build your own (excluding the rhombic).
Optimization is nor very practical unless you want to spends 100's of $$$ on
test equipment. The large Radio Shack FM antenna, if they still make it,
should do the job. An ANTENNA MOUNTED preamp might also help. Since you are
trying to pick up distant stations, antenna height is very important.

Tam/WB2TT