Air to Ground Communications
2182 Analysis
DGPS Antenna Analysis

 

SHUNT FED VERTICAL

The shunt fed vertical antenna at first appearances looks rather plain and simple. However the performance characteristics of this antenna are EXCELLENT. It is a quarter wave vertical, with a quarter wave of counterpoise radiating out in a 360 degree radius from the center of the antenna. Using a OMEGA MATCHING device found at the base of the antenna, with a Rohn 25G tower, we have found that the tower/antenna will produce about a 800 KHz bandwidth with a VSWR (Voltage Standing Wave Ratio) of 2 to 1 to 2 to 1. This VSWR curve is nice and linear. I have one in my back yard that is used for amateur radio 40 meters (7.000 MHz to 7.300 MHz). It tunes through those 300 KHz of assigned frequencies with almost a 1 to 1 VSWR?

Verticals are know to be "noisy". Yes, I agree they are, when compared to a beam antenna or a delta loop. However, they are not as noisy as you might think, especially when receiving frequencies within their transmit range. They have good receive ability, but not as good as the conical monopole.

The shunt fed vertical excels is on transmit. It provides maximum electrical efficiency, and is able to reach "unity" (when compared to a dipole) if the counterpoise is adequate. The optimum counterpoise is a solid sheet of aluminum surrounding the antenna for a quarter wave in any given direction. However, that is not possible. Thus, 100 radials and 8" aluminum flashing will do. If possible, some of the ground wire will be attached to water pipes and chain link fences in the surrounding area. This will give the end user a tremendous punch when transmitting. Same is true on the receive side, as long as the receiver is tuned within the frequencies of the transmit range.

The antenna in my backyard for 7 MHz, is 9 meters high (30 feet). Through the OMEGA match, that antenna will load on 8 MHz or 9 MHz with few problems. It is just a matter of changing the capacative values. If the end user would rather use a "coupler" or automatic antenna tuner, the antenna will load from 2-30 MHz. If a customer wishes the 2-30 MHz version of this antenna my first question is: How high can I build the antenna for your location (optium height for the 2-30 MHz is 30 Meters or about 110' high), with a 30 meter counterpoise in any given direction around the base. So now we are talking about 1 acre of land, with appropriate phillistran guy wire (non-radiating fiber cable which has the tensile strength of metal cable). If you require this antenna to be mounted on a roof, then I suggest my capacative hat vertical (about 50 feet high) with a capacative hat at the top. However, to insure that the electrical efficiency is adequate a 2 MHz+ antenna must have sufficiant counterpoise. That does not mean just mounting it to the metal superstructure of the building. A vertical antenna is a tuned circuit, and needs a quarterwave counterpoise beneath it at a 90 degree angle from the radiating surface.

Here are some photos and VSWR graphs for your viewing pleasure.

ANTENNA COUPLING DEVICE

Machined Coupler

OMEGA MATCH

One capacitor is in series and the other is in parallel.

FEED POINT

Shunt Fed Vertical Bottom

Base of antenna with Omega Match and Gamma Rod

GREAT PHOTOGRAPHY!

Shunt Fed Vertical Upper

Antenna on the job.

VSWR CURVE, HF

VSWR shunt-fed graph

Once the antenna is tuned, it load wonderfully. The VSWR curve may change depending on the dryness or wetness of the soil. That is why one should put down as many radials as possible. This single antenna could be retuned for 6 MHz or up to 8 MHz, just by adjusting the OMEGA matching system.

 

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