I have a couple Midland cb radios, model 70-1066. All the info on these have been lost. I need to build a base station type antenna for these. I need the transmission frequency and antenna impedance to do this. Maybe a complete design would be nice. Than you.

 

The channel frequencies - http://www.firestik.com/Tech_Docs/CBFREQ.htm

The output impedance should be 50 ohms.

 

Thank you, now I have to find out how to construct a dipole for 27MHz.
I will look at the sites that Bertus mentioned.

 

I have just been told that the frequency is actually 80MHz
Any ideas on a dipole design???

 

Here is a link to lots of stuff - http://www.ac6v.com/antprojects.htm

 

In your country of origin it could be different, but in the USA CB is 27Mhz. Always.

There is another standard called FMS, a FM band public system. Again, this is USA standard.

 

Thanks Bill,

It used to be the case (27MHz), but now 54MHz is allowed and things like 80MHz can be used by getting a licence from the relative authority. It is much like a Ham-licence, but no exams. Strange but tru, this is Africa.
Enjoy

 

A lot of countries, like Mexico, are very paranoid about 2 way radios. The USA has relaxed a lot of standards over the years. CB in the USA used to require a license, but in the early 80s the FCC gave up on it, because the $10 fee was deemed unreasonable by the courts (I think).

So 27Mhz is no longer legal in Africa?

GMRS and FRS have mixed requirements on licenses, FRS doesn't, GMRS does, and they overlap, which is very confusing. Neat walkie talkies though.

 

27MHz and 54MHz are the supposedly legal frequencies. But like the 80 MHz radios that I am busy with, you apply for a licence and pay a little money and there you go. (You actually get issued with a Call sign{number) like a ham radio.) These are fully portable 12Volt motor vehicle type units from Midlands and nothing stops you from driving around and talking to a similar unit in the area. The best answer I can get out of Midlands "South Africa" is that the type number 70-1066 means it operates in the frquency band 66 to 88 MHz. According to you guys, that should be a no-no.
Anyway, the plot goes on. I still need to know how to make a dipole for the 80MHz band.

 

At VHF frequencies you need to be concerned about polarization. If you have a dipole, which is horizontally polarized you will have trouble talking to a guy with a vertical antenna.

 

Anyway, the plot goes on. I still need to know how to make a dipole for the 80MHz band.

468 divided by the freq. you want to use will get you there.

 

Wavelength = c/f where c is the speed of light, 3*10^8 and f is 80*10^6.

This is 3.75 meters. For a quarter wavelength this is 0.9375 meters.

 

When making an actual antenna, the length of the elements will usually need reducing by about 5% from the theoretical measurements to allow for 'end effect'.

Basically, the frequency shifts due to the capacitance of the antenna elements and the resonant frequency is lower than that calculated by wavelength alone.

 

I guess the OP may have lost interest, but if I were building an antenna for 80MHz, rather than a horizontal dipole, I would build a vertical that looked something like this.

http://cms2pdf.com/procom.php?id=489&name=GP80&language=eng&style=low

Note that there is a graph on the page to calculate element lengths. A 1/4wave ground plane is as simple antenna, but that's what seems to be needed.