Advice for antenna setup please

packerfan11 · Sep 7, 2009

Hamster said: ↑

I have to contradict you. The antenna is one of the most important components of a transmitter setup, since it is converting the RF alternating current in the coax cable into the electromagnetic field.
You want as much as possible of this energy be converted into electromagnetic radiation, which the other guy can receive. You don't want to waste this energy by producing heat.
A good antenna setup is the best and cheapest HF amplifier, both for transmission as well as for reception.

The basic rule is to get the antenna resonant (yes, tune it like a guitar) for the respective HF band, get the SWR matched (goes together with the first demand), to get as much wire into the air as possible (though it must correspond with the first demand) and to get the antenna set up as high as possible.

For a whip antenna the best place is the middle of a metal vehicle roof (which is used as as a counterpoise).
The length of the antenna should be preferably be 1/4 wavelength minus a shortening factor depending on the thickness of the antenna wire and surroundings (capacitances and inductivities of the surrounding structure). This means an antenna length of about 6 3/4 feet and is a requirement given by the resonance demand.
You can mechanically shorten an antenna using coils (many commercial CB antennas have loading coils installed at the footend), so that to the current the antenna still has the same electrical length (by increasing the inductivity), but you'll loose efficiency and waste energy.
Especially when you are working CB with the low powers and insensitive receivers involved , you can't afford to cut corners with the antenna.
And BTW, you don't need an expensive fancy brandname antenna, a stiff piece of wire cut to the right length, mounted in a good antenna mount will do.
Important is also the connection where the braid of the coax cable meets the metal surface. The braid connects to the outer body of the PL plug. The antenna mount has to be neatly grounded to the metal surface at the antenna feedpoint.

Concering the SWR, a properly setup 1/4 wave antenna will have an impedance at the footpoint of about 50 Ohms. This is what the transmitter expects to see so that it's final stage can work best. Any other impedance will cause HF energy to be reflected back into the transmitter, causing it to overheat.
Now the impedance (AC resistance) of an antenna is being made up out of two components:
the radiation resistance, which is a fictive resistance and shows the amount of energy actually converted into radio waves, and the parasitic resistance, which is caused by the amount of energy converted into heat.
You'll want to get the radiation resistanceas close as possible to 50 Ohms, which will happen if the antenna is in resonance, and the parasitic resistance as small as possible to get most of the energy into the air as RF waves.
A 50 Ohm resistor connected to the transmitter ouput will give you a perfect SWR of 1, but it won't radiate any HF energy, just produce heat.

In my CB days I had connections from a hilltop in Germany all the way to England, Italy and Slovenia in good propogation conditions during summer, using the legal 4 Watts and a roofmounted antenna with a magnetic base (I didn't want to drill a hole into the roof of my car back then). This means I have covered distances of more than 1000 miles.
I just made sure that I had the antenna set up as good as possible.

A fellow ham nearby has a sideline fixing and maintaining the CB sets of the fleet of trucks of a nearby rock quarry company, which owns several rock quarries in the county. They got rid of an expensive commercial system after they increased their business and instead bought CB radios from him. Since the antennas are subject to hard wear, he did just get standard antenna mounts and made up the antannas themselves out of heavy gauge steel wire. BUT, he took the efford to calibrate each antenna individually using his sophisticated testing equipment to get maximum performance. And he wrote up all antenna dimensions. So if they loose an antenna, e.g. by brushing a tree branch, he will just cut a new piece of wire to size and fit it. The quarry truckers routinely talk to each other over distances of 15-20 miles.

Jan
Click to expand...

Right on Hamster...Someone who gets it.

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