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#13753 - 09/11/02 04:48 AM Aerial(antenna) Length's  
Trumpy  Offline

Joined: Jul 2002
Posts: 8,223
SI,New Zealand
I am an electrician here in New Zealand,
I am also a Radio Amateur Operator (ZL4TPQ)
Could somebody tell me if there is a
formula(e), for calculating the length's
of the elements, as well as the boom spacing,for 26MHz, over here, yes I realise
that this is the CB band over here,
this could be for a 3,4 or 5 Element Yagi,
But, I am building this antenna for a
mate, George Corran, can you help?.

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#13754 - 09/11/02 09:09 AM Re: Aerial(antenna) Length's  
Chris Rudolph  Offline
Joined: May 2001
Posts: 160
Winter Park,Fl USA
I believe that the info you are looking for can be found in the amateur radio handbook.
I am sure that you recall that you have to match the transmission line(50 ohms to free space 377 ohms)and the relation for antenna element length,number of elements and spacing is a function of frequency and desired beamwidth/gain.Sorry I cannot provide you the info off the top of my head.
There could be a Web page that gives the design data.Good luck.

#13755 - 09/11/02 09:28 AM Re: Aerial(antenna) Length's  
George Corron  Offline
Joined: May 2001
Posts: 717
Lorton, Va USA
Sorry, we're a bit busy at the Pentagon today. There are 2 basic types of antennas (OK there are literally thousands, but 2 basic types everyone uses)

The most common is the dipole. This is a piece of wire, great antenna and with a few minor "tweeks" is what I run. The formulas are fairly simple for this one.

You design antennas in increments of wavelengths.
Full wavelength = 984/frequency in MHZ, or in your case 984/26=37.85 feet
1/2 wavelength = 468/frequency in mhz or 468/26=18,
1/4 wavelength = 234/26= 9 feet.

A Yagi antenna (looks like your typical TV type) is made up of at least 3 elements. The one in the middle is called the "Driven" element and is usually 1/2 wavelength long, in your case, as above it would be 18 feet. The driven element is the one you connect your receiver transmitter to.

The long element behind the driven element is called the "Reflector" and is longer than the driven by some mathematical coefficient, most of my friends use 10%, so in this case it would be 18 feet plus 10% or 19'8"

The short element is called the "director" and like the reflector is a mathematical reflection like the reflector. If you use 10% it would be 16'6".

You can compound the Yagi with more directors and reflectors and it will help a bit.

Yagi's of this size we find aluminum is great, but feel free to manufacture out of bamboo with the copper wires taped/ty-wrapped to it.

Hope this gives you some direction, of course I'd never transmit without checking and really fine tuning with an SWR meter, and I'm certain you realize this. Also, these things aren't nearly as critical for only a receiver, again I'm certain you know this, but for others who may wander in.

Lemme know if you need anything further if I can be of assistance.

George AF4JH

#13756 - 09/11/02 04:30 PM Re: Aerial(antenna) Length's  
pauluk  Offline
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Both the ARRL and the RSGB handbooks have some pretty comprehensive coverage of this topic. I can't get at mine at the moment as they're buried in one of about two dozens boxes stacked in one bedroom while I'm decorating another! But I think George has covered it fairly well anyway. [Linked Image]

By the way, for those who aren't aware of it, the NZ CB allocation is slightly lower in frequency than in the U.S. and most other places. Trumpy, Jaycar catalog gives it as 26.330 for ch. 1 thru 26.770MHz for ch. 40. Is that right?

#13757 - 09/13/02 02:38 AM Re: Aerial(antenna) Length's  
Trumpy  Offline

Joined: Jul 2002
Posts: 8,223
SI,New Zealand
Yes, Paul,
You are correct there,
I think that NZ is the only country,
that uses this particular frequency range,
don't know why, but I have a feeling,
so that communication with Australia,
is not possible.
But during, the skip season of 1995,
I could talk to CBers in Canada and
to a certain extent, some operators from
the US, go figure, and all this on
12 watts PEP (SSB).

#13758 - 09/13/02 04:36 PM Re: Aerial(antenna) Length's  
pauluk  Offline
Joined: Aug 2001
Posts: 7,520
Norfolk, England
I don't know about NZ, but in many parts of the world you'll find plenty of illegal operation both above and below the standard 26.96 - 27.41MHz band.

Reception varies tremendously by conditions and frequency in use. During the last high sunspot period I could regularly hear American CB operators from the deep South. Some were running a little more than the legal 4W of AM, but many were just regular legal sets. The 10-meter amateur band being so close also behaves similarly.

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