GAP Verticals - The Somewhat Different Shortwave Antenna
A short excursion into the theory off vertical antennas:
A common vertical antenna consists of a vertically mounted metal rod with a base loaded feed and a certain number of concentric radials. In order to enable multi-band operation traps are inserted at appropriate locations that effectively adjust (=shorten) the antenna's electrical dimension to a quarter wavelength
So far so good; but where is the radio's power going?
Lets assume we use an antenna elevated by 8 m. For 80m this antenna should be elevated by 20m above ground in order to fulfill the 1/4 λ resonance case. The "missing" 12m are substituted by using traps. Now the antenna is electrically 'too short' resulting in an antenna resistance of approx. 4 Ω. The 4 Ω represent the virtual resistance that effectively radiates the RF energy.
The antenna resistance, unfortunately, is not the only one resistance in our antenna system. The most important resistance is the so called earth loss. The earth loss depends on the soil characteristics as well as on the radials.
If, for instance, three radials are used for the antenna the resulting ground resistance is in the order of 30 Ω. Our antennas system now consists of a radiating antenna with 4 Ω and 30 Ω ground resistance in series. The ground loss will very much heat up the surrounding soil! In total the system has 34 Ω. However, only 4 Ω contribute to the antenna's radiation! So if the radio feeds 100 watt into the antenna system only 12 watt will be radiated. If one adds up the radiation losses of the traps required for multiband operation with 2 Ω only 11 watt will be the effective radiated power.The earth loss contributes the most significant portion besides the traps.
Solution to this problem: Easy, just use many more radials!!
Okay.... take 1000m of wire and create 60 ground radials: This will reduce the earth resistance from 30 Ω to 4 Ω; not bad, but still the radiated power is only 50 watt and the action taken was massive!
Still one half of the RF energy warms the ground!
In order to significantly improve this situation a new antenna technology must be used.
GAP antennas elevate the feed point. Advantage of the elevated feed point design:
- Radiation resistance increases as the feed point is elevated. If the proper elevation is selected, the radiation resistance will be 50 Ω - a perfect match to the feedline.
- Elevating the feed point also virtually eliminated the earth loss. The GAP antenna has an antenna resistance of 50 Ω and an earth loss of 5 Ω. This efficiency of approximately 90% creates an antenna with very little loss; if we take the above example 90 watts from 100 watts input will be radiated. The hot spot marking the maximum antenna current is now located close to the antenna center section instead of being close to the earth. Result: The RF is radiated rather than used to warming the ground.
- The resulting antenna gain for low take-off angles increases.
GAP antennas feature a high efficiency:
- no earth loss
- no loss in traps or transformers
- the entire length of the antenna contributes to RF radiation. Trapped antennas contribute only with a part of their mechanical length to radiating on the higher bands.
There are no traps in a GAP antenna, no coils nor transformer.
Here are five reasons to avoid traps:
- when not installed properly traps may be prone to take on water
- the coil portion of the trap changes its value as of function of temperature shifting the operating frequency of the antenna.
- a trap is a coil capacitor that operates at high voltage; high voltage in a moist environment is prone to arc and self destruction.
- a trap must have a high 'Q' to perform efficiently; a high 'Q' will result in a narrow band width and restrict the antennas band width.
- finally, as everyone knows, traps are lossey; they steal transmitter power.
GAP antennas don't require tuning.
There is absolutely nothing for you to tune.... In fact, as trip after trip to readjust the antenna on the roof or on the mast raises your frustration level and you may loose your "cool"... All GAP antennas come completely electronically pretuned from the factory.
GAP antennas have a high band width.
With only a few exceptions the GAP antennas cover all ham bands featuring a SWR of <2:1.
Quiet - please.
Noise is an unwanted companion of verticals - particularly on the low bands. All GAP verticals are"quiet" antennas primarily due to a sleeved feedline and the use of a counterpoise. GAP antennas eliminate the deployment of thousands of feet of radial wires "parallel to" the AC power lines which transfer power line noise.
GAP antennas are easily installed.
Just screw the nuts and bolt into the pre-drilled holes with the provided tool!
GAP antennas are built to last
All weather performance should be important to you. Simplicity and a minimal parts count are the key elements to reliability. Consider the elements of the GAP antenna - aluminum tubes and coax. Nothing else! Compare that to other antennas with a multitude of connections, coils, variable capacitors, transformers, etc. and you'll see that for everyday operation in rain, sun, ice or snow - GAP works. GAP antennas use double-drawn aluminum tubing. Each section telescopes perfectly into the adjacent and is secured with screws. The hardware is stainless steel. The coax used is a special non-contaminating high temperature type to provide added safety for high power operation.
Items 21-23 of 23
Items 21-23 of 23