G8MNY > TECH 10.05.22 07:05z 79 Lines 2380 Bytes #24 (0) @ WW
BID : 1923_GB7CIP
Subj: A simple 3 ele Yagi for 144MHz
Sent: 220510/0702Z @:GB7CIP.#32.GBR.EURO #:1923 [Caterham Surrey GBR] $:1923_GB
To : TECH@WW
From: G0FTD@GB7SXE.#38.GBR.EU (Updated Mar 08)
Here is a simple 3 ele yagi for 2m /P work which I made up from an old
Band 2 FM antenna with elements about 5mm diameter.
_____________________ 976 mm
Distance = 300 mm
___________o___________ 1000 mm
Distance = 590 mm
_________________________ 1004 mm
The reflector & director are touching the metallic boom in the centre.
A coaxial matching stub is used to match the driven element to 50ê due to
the coupling effect of the other elements. Attach a length of approx 20cm
RG58 coax at the feedpoint of the dipole (in parallel with the feeder cable
& leave it open ended).
Some trimming will no doubt be needed of the matching stub for best match.
I find it best to start off with a slightly longer length than required &
trim by *5 mm* each time. It is essential to trim by 5mm because the exact
point of matching is quite narrow.
(I recently changed from a coaxial stub to a 5-15pf trimmer across the)
(feedpoint. This too works a treat. - Andy -)
For UK readers I've also purchased some of the old style Beehive trimmers,
0-30pF for 10 pence each from WH Westlake. They do the job too, and are
great for making VHF/UHF ATU's if you are that way inclined.
Earlier on I scribbled some figure in my notepad about element size on
this 3ele design.
With elements @ 2.5mm diam = 7.34dBd gain
With elements @ 5mm diam = 7.01dBd
With elements @ 10mm diam = 7.42dB
R changes by about 2 ohms but j varies a bit more.
Not a problem with a coaxial stub or capacitor across the feedpoint.
You may vary the length of the driven element slightly & it will not impair
performance, but you will have a slightly different length needed for the
tuning stub or value of feedpoint capacitor.
As a matter of interest, I entered the Practical Wireless 2m QRP Contest
with this 3 ele yagi attached to my radio trolley, all of about 1.5m
above ground level.
I came 54/80, however consider these two factors:
* The leading single operator used 22 elements compared to my 3 ele.
* I only operated for 1 hour out of the 6 hours (I think it was 6!)
that the contest runs for!
Full details in the Practical Wireless November 2007 edition.
- Andy -
Why Don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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