RDF Yagi Antenna for FRS Radio

[charlienorth]

Hi All

I am afraid that I know nothing about RF. May I appeal to you
for help with this project?

I bought 4 FRS radios and am about to equip a few rockets with
them in a manner similar to this:

http://www.nakka-rocketry.net/frs.html#top

The radios that I have are capable of continuous transmission so
the 555 circuit will be replaced with a buzzer mounted near the
microphone.

I plan to make a RDF Yagi like this:

http://home.att.net/~jleggio/projects/rdf/tape_bm.htm

I plan to use YagiCad 4.1 to design the antenna.

http://www.ipass.net/teara/kq3.html

My (first) question for you is may I design for the middle of the
FRS frequencies, say 465 Mhz, in an attempt to be able to use
them all or must I design it for one specific frequency?


FRS Channel No. (MHz)

1................... 462.5625
2................... 462.5875
3................... 462.6125
4................... 462.6375
5................... 462.6625
6................... 462.6875
7................... 462.7125
8................... 467.5625
9................... 467.5875
10.................. 467.6125
11.................. 467.6375
12.................. 467.6625
13.................. 467.6875
14.................. 467.7125

 

[charlienorth]

Thank you very much.

I'm glad you've used the program successfully.  It sure looks like a
novice like me possibly could make it work.

I'll use the YagiCad program to design for 465.13 Mhz.

I understand that I am looking for a foward looking pattern as shown
here:

http://home.att.net/~jleggio/projects/rdf/tape_bm.htm

It also seems a low impedance is important but I also read that it is
very important to match the feed impedance to the antenna impedance.

Sorry to say, I have no clue how to do that. I do have experience
destroying things by not matching impedance. (Hooked a sound card to
a new boombox, argh 

 

[charlienorth]

Thanks again for your reply.  You are not putting me off at all.
I need to learn at least a little of this and thanks to you I learned
a couple new terms.

Yagi Cad says this design that seems good and that ithas input
impedance 11.6 +j 0.0 Ohms

I can research how to make a balun but I'm not sure what it is that I am
to match?  You promised the program would tell me. 


Here's what I input and the results from YagiCad in meters/mm:


465.14
3
20.32
20.32
20.32
20.32
20.32
.3462315
.3064886
.289976
.2571061
.244101
5.631533E-02
7.568619E-02
.1289118
9.078011E-02

 

[charlienorth]

I looked up baluns in the 1993 ARRL Handbook.  The bazooka balun
(Figure 14B, page 16-7) looks like it will balance the impedances if the
bazooka sleeve is 1/2 the length of the driven element.  Have I got that
right?

 

[charlienorth]

Oops, I care about receiving not transmitting right?  (And the balun
reguards matching antenna/transmitter impedances?)

"Coupling" the receiver to the antenna is my concern correct?

 

[charlienorth]

I am back to realizing the impedances need to be matched.


--------------------------------------------------------------------------------

>Scott Fintel said:
>Most all of your TV antennas use them to go from 300 ohm leads
>on the antenna to 75 ohm cable.

And tekwiz at the Electronic's Forum just told me that is exactly what
is needed th balance the transceiver/antenna impedances on the FRS
RDF setup.

http://www.dutchforce.com/~eforum/index.php

BTW, Joe Leggio was kind enough to reply to my query about designing
the Yagi for FRS radios with the advice to scale all dimensions down,
including the element (tape) diameter, with the following ratio:
Scaling factor = 145.565/465 Scaling factor = 0.313

 

[charlienorth]

Thanks again for your response. 

In recommending the bazooka, do you feel that it is superior to the
cheap Radio Shack type 75 to 300 ohm units? 

 

 

[charlienorth]

I bet your Tonna antennas would be good for direction finding!  I guess you'd
need a tank and 40 acres to turn the things though. 


If I am understanding what you mean, the 1/4 wave bazooka or the 1/4 stub
would seem to cost the same (almost nothing) since the pieces needed are
virtually identical.

By 1/4 wave stub, you are meaning the design that has the same tube as the
bazooka but is oriented to the side of the feed and attached to both the
shielded ground and the center feed at the antenna?