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VK3ABK > 50MHZ 07.04.07 06:41l 66 Lines 2803 Bytes #999 (0) @ WW
BID : 54098_VK3HEG
Read: ON4CBL GUEST
Subj: More beam design.
Path: ON0AR<IW2OAZ<SR1BSZ<CE8FGC<ED1ZAC<IK8YAP<IQ0LT<ON0BEL<VK7NW<VK3HEG
Sent: 070407/0522Z @:VK3HEG.#WEV.VIC.AUS.OC #:54098 [Ballarat] $:54098_VK3HEG
From: VK3ABK@VK3HEG.#WEV.VIC.AUS.OC
To : 50MHZ@WW
Hello G0FHM and others interested.
James has asked....
"Does anyone have a "quick n durty" beam design?, something that is easily
knocked up in a short amount of time, and with relatively easy to find
materials?"
With respect to john, G8MNY, who has repeatedly sent antenna designs in his
packet bulletins, "quick n durty" may not apply, but his 50Mhz antenna from
an old TV beam may do the trick. See '6m Yagi from TV Antenna', Or don't you
see John's frequent bulletins in the UK?
How about a rerun, John?
And I see a bulletin from Bob, VK6BE, who gives details of a 6m Quad antanna
with rederence to the ARRL HB for dimensions. But surely we can do this by
starting from elementary (!) principles. It' not hard to design an antenna
with a basic understanding of EM propogation physics, and anyone can design
their own beam, or any other antenna with good results.
The lengths of elements, or wire in dipoles or quads, can be calculated
knowing the 'speed' of EM waves and the frequency you are interested in.
Then using the equation....
L (of a half wave dipole) = 300/f divided by 2
Where 300 is the speed (divided by 10^6)
And f is the frequency (divided by 10^6)
(This gets rid of a lot of zeros)
This means that we are taking the EM wave propogation speed as a fixed entity,
and dividing by the operating frequency in MHz to get a wavelength.
This equation will give a half wave length in space, so by making a 5 percent
reduction, allowing for conduction in a solid medium, and 'end effect', you
will have the length of your driven element. This applies to beams and wire
dipoles, but not so much in quads as they don't have ends!
For a beam, (or quad) the reflector could be 5 percent longer than the driven
element, and the first director about 5 percent shorter (for a wide spaced
3 element beam).
So, forget your various formulas for lengths, they are usually for feet and
inches and only do what is easy from known theory. 'Looking up' is just a
crutch and the various design programs just use principles, with the added
advantage of quickly comparing designs. For a one-off, you don't need that.
Spacing between beam elements is not critical for small beams at HF and low
VHF. One quarter wavelength is a good starting point, but for a beam with
many directors, a closer spacing of the first and second directors forms a
'launching section' which best couples it to the rest of the beam.
This has been my experience, with theoretical and practical experience, and it
is an important part of Amateur Radio that can still be a source of experiment
and interest. Well worth giving it a go.
73. Dick. VK3ABK.
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