WA Radio Scanner Users' Group

Hey guys,

I am currently making a 1/2 wave dipole for 27MHz CB and I am having a few issues. I cannot get the antenna SWR down to an acceptable level. I am aiming for a frequency of 27.355 (CH35). (300/27.355)/4 = 2.74m (Length of each side of antenna). I am using RG58U for the antenna itself and will be using rg8 for the feed line to the top of the roof. I did, however, use RG58U as the feed line when testing in the office. I also built my own centre connector, with one side connected to the feedline shield and the other side to the centre conductor.

I cut the required length, soldered it all up, I laid the antenna on the office floor, hooked it up to the radio, but when I test the SWR it is sky high. Near the top end of the scale. I then started nibbling bits of coax off each end to tune the antenna but the SWR was not improving.

I am new to building antennas. Can someone tell me what I am doing wrong???

Dipoles need to be up in the air when you test them. That is probably having the most effect in your test. Try to get it at least 1/2 wavelength up. Preferably more

A dipole is a balanced antenna where-as coax and the radio are unbalanced so you should use a balun (you can make a simple choke balun from coax... google helps there)

How did you wire the coax being used for the radiators? If you wired the centre core only then you should remove the shield.

Another thing to take into account is that a dipole is open ended so you need to allow for "end effect" (something I don't understand but still do anyway) This means you need to shorten the radiators by about 5% even before you start trimming for SWR. Closed antennas (loops, folded dipoles etc) don't have end effect.

I thought that baluns weren't required for dipoles, unless there was excessive RF travelling down the feed line...

I constructed a sealed unit for my centre connector.

Is that how it's meant to be?

So, taking into account end effect, the calculation would be as follows:

(300/27.355)/4 = 2.74m
2.74 x 0.95 = 2.60m *Length of each radial

Then, start tuning from there?

You could try a RF choke made from 10 turns of RG58 on 10mm white PVC pipe. This goes up near the center connector.

http://www.hamuniverse.com/balun.html

Always use a Balun or RF choke so the neighbors don't come banging on the door with RFI complaints.

EDIT :
Looking at that picture using RG58 as the antenna wont work, the outer shield will attenuate the signal.

Haha, duhhh. I never even thought of the bloody shield!! Is my centre connector good? What are some good alternatives for the antenna wire?

Just short the inner and the outer at the sockets leading the the radiating element and the counterpoise
Nick

Reevesy89 wrote:Haha, duhhh. I never even thought of the bloody shield!! Is my centre connector good? What are some good alternatives for the antenna wire?

I bought a roll of earth wire from Bunnings as it blends in with the trees.

Is there a min. gauge of wire that needs to be used?

The thicker the wire, the wider the bandwidth. At 27mHz, not all that important.
Nick