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60wag
02-01-2009, 07:24 AM
So this is my CB antenna not my ham but I'm puzzled. I was finally doing a permanant mount for the CB so I wouldn't be using a magnetic base and I broke the coil of the base loaded antenna. I took it apart to see if it was salvagable and was stumped by the apparent construction:

The center pin that threads on the whip is attached to a coil that is also attached to the center pin that threads onto the mount stud center pin - makes perfect sense.

Then there is a second coil with about 3-4 turns that is connected directly to the ground shell of the base connector. This looks like a direct short from the center pin to the shell. The coil is made with the same heavy gauge, low resistance wire that the longer coil is made from.

The antenna is a cheap Radio Shack thing that will likely get pitched but I would have expected a complete separation between the ceneter pin and the sorrounding ground. Any ideas?

Groucho
02-01-2009, 08:24 AM
My guess is that it could be a shorted to ground antenna. Our homebrew 2M antenns are shorted to ground at the base of the coil. It is an inductance match to get the antenna to 50 ohms.

What does this accomplish and why?

First, DC grounding helps control static discharges from the antenna, thus reducing some of the received hash we all put up with. Secondly it is a safety issue. If the antenna were to come into contact with a live overhead power line, DC grounding will help prevent damage to your equipment and perhaps to you as well. Newcomers might think that a DC grounded antenna won't work, but this is not the case. Just because we DC ground the antenna, doesn't mean it is RF grounded too. There are several ways to accomplish a DC grounded, RF match.

Basically we are impedance matching the antenna to get it close to the radio's impedance of 50 ohms. Matching is achieved by borrowing a small amount of capacitive reactance from the antenna (when tuned slightly above the operating frequency), and the inductance of the coil. Together, they form a highpass, LC network which transforms the antenna's low impedance (typically 25 ohms or so) to that of the 50 ohm feed line.

This may or may not be the case with your antenna. Antenna manufacturers have been known to do some crazy things.

DaveInDenver
02-01-2009, 11:07 AM
Remember that DC ground does not also mean AC ground. A coil that is connected seemingly directly to ground will have resistance to ground of something very low (say single digit ohms), but to RF that coil might show 100, 1k, 1M ohms of impedance. Like Groucho says, this is done often to dissipate static (mobile antennas can build up significant static charge driving down the road) or also extremely often to build an impedance matching network.

Two common ways to impedance match are with a transformer like on the left or a smaller and simpler (this more better for mobile antennas) using the network on the right. The lump elements of L, R and C represent the characteristics of your antenna itself. Z is the feedpoint impedance, which will match the coax and transmitter impedance.

11866

But notice how n2 will most certainly appear like a short to a DC resistance meter. Also it could look like a short or open at difference frequencies, which is the point of a matching network, to make the impedance at the feed point be close to 50 ohms at the range of frequencies that the antenna is tuned for.

60wag
02-01-2009, 05:22 PM
Thanks for the explanation. I thought is was an RF thing but had a tough time getting past the thick wire shorting the antenna to ground.