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Beverage Antennas Construction by Tom, W8JI - part II.

15. listopadu 2006 v 17:25 | Petr, OK1RP |  Antennas
Multiple Antennas Crossing
Crossing of Beverages has little effect if they are not parallel or nearly parallel. Try to cross at an angle of 90 degrees if possible. Even a few inches of spacing is enough for right angle crossing. With shallow angles, assuming they can not be avoided, increase wire spacing to a few feet.

Transformers

Always use isolated transformers for feeding Beverages. It is cheap, simple, easy insurance against unwanted common-mode ingress of noise and signals into the antenna from the feedline shield. See the Common Mode Noise page for an analysis.
I use 73-mix FairRite Products 2873000202 cores (about 1/2 inch square and 1/3 inch thick 73 material) in my transformers. These cores require a two-turn 50-75 ohm winding. The high-impedance winding is 5 turns for 75-ohm cables (6.25:1 Z ratio) or 6 turns for 50-ohm cables (9:1 Z ratio). Small insulated hookup wire is actually better than enameled wire. The thicker insulation is much less susceptible to developing shorted turns in rough service.
While my early transformers were waterproofed with Krylon and coated with insulation foam, I have finally laid out enclosed transformers and terminations with internal lightning protection. The transformer sections have F-fittings, and all use stainless steel hardware.
For a Reversible Beverage, I use the following:

Multiple Antennas at One Feedpoint

Never bring multiple antennas to one feedpoint, especially when they share one common ground. I've noticed a definite deterioration in pattern with multiple feedpoints arranged with only ten feet of spacing, even when they had separate ground systems. One set of Beverages installed with 5-10 foot of feedpoint separation has noticeably poorer patterns than other identical length antennas with wide separation at the feedpoint.
Multiple antennas actually may be the only case where a sloped feeder can make a difference, the slope will actually move the effective feedpoints further apart. The best idea, however, is to separate the feedpoints by several times the antenna height.

Termination Value

Having precise termination values isn't necessary, but get as close as you reasonably can. There are some impedance measurement suggestions circulating that absolutely do not work. One is to just use a tuner to match the terminated (or unterminated) antenna, and work backwards with loads to measure tuner impedance ratio after matching. This won't tell you a thing about proper termination, unless you repeat the measurements on dozens of frequencies spread over a wide range!
There are three fast, simple ways to test for proper termination:

With an Antenna SWR Analyzer

  1. Connect the antenna analyzer at the Beverage feedpoint through a good matching transformer
  2. Sweep the analyzer frequency from 1.8 to 7 MHz (or over a ~4:1 frequency range near the frequency intended for antenna) while watching SWR
  3. Adjust termination for minimum SWR variation (not minimum SWR, minimum SWR variation!)
When installation (including grounds) and termination is proper, SWR VALUE will remain nearly the same regardless of frequency

With an Antenna Impedance Meter

  1. Measure the feedpoint impedance (right at the feedpoint) of a roughly terminated antenna at the frequencies of highest and lowest resistive impedance. You can do this through a known good transformer by correcting impedance for use of the transformer
  2. Multiply the lowest measured impedance by the highest, and then find the square root of that number. This will be the correct termination impedance of the antenna

With a Clamp-on RF Current Meter

(This does not work well with voltage, because of measurement method error problems)

  1. Apply a small amount of power from a transmitter, do not exceed antenna system component ratings!
  2. Measure current at the termination, and several points up to a distance of at least 1/2 wl from the termination
  3. Adjust termination resistance so current shows a smooth current decline as you move the meter towards the termination
Note:
In about 500-800 feet of distance, power loss in a Beverage is around 3dB. This corresponds to a 1/3 reduction in current. If you attempt to adjust for equal currents (or voltages) over any distance, the antenna will be MIS-terminated!

Identifying a Composition Resistor

We commonly assume any brown phenolic resistor is a carbon composition resistor, but that isn't true. Most of the smooth brown-colored phenolic cased resistors manufactured after 1960-1970 have actually been carbon film resistors. There are only a limited number of manufacturers supplying carbon composition resistors. One is Allen-Bradley. They are expensive special-order parts, and the buyer must specify composition types.
As we see from the photo, it is impossible to identify a composition resistor by external appearance.
The only sure way to identify a resistor, short of ordering it from a reputable source, is through a destructive test. We can, for example, apply a large momentary overload and look for a resistance change. A resistance change indicates a film-type element. We could also cut the resistor open, and look for a non-conductive core. A non-conductive core indicates the resistor is a film style component.

Why Composition Types?

We need composition resistors in any application where the resistor is subjected to very-large very-short overloads, or where the system demands a nearly pure resistance at a very high frequency (F>100MHz).
Obviously, in the case of a Beverage at a few MHz or lower, we could get away with using many styles of wire-wound resistors or spiral-film resistors. A small amount of inductance would not be a major problem, and virtually ALL carbon or metal film resistors (constructed with resistance elements deposited or cut in a spiral on an insulated core) would not have excessive inductance. The thing we can not tolerate is the sensitivity of non-surge rated components to damage from lightning storms, even distant storms.
The life of a carbon or metal film resistor, when used as an antenna termination, is relatively short in most locations. Just a few coulombs of energy, when applied in a few milliseconds, will cause a carbon or metal film resistor to change value. Worse yet, the resistor will not be altered in appearance. (Carbon also has a strong tendency to change value with heat. Even modest operating temperatures, over a period of time, will cause a carbon resistor to change value. Metal resistors are more stable.)
Unless you want to make a full-time career out of testing your antennas and replacing resistors, use a energy absorbing composition type resistor!
I install a small lightning gap of about 1/8th inch across my antenna's ends, both at the feedpoint and the termination. This helps immensely with very close strikes. I use either Ohmite OY-series metal compositions or A-B carbon composition resistors. You can buy metal composition resistors at DX Engineering.

Ground Systems

The ground system mainly provides an RF and lightning ground. Having a very low ground-resistance is not especially important, unless an Autotransformer or Un-un is used! Autotransformers and Un-un's don't isolate the feedline for common-mode. The antenna needs a stable ground, not necessarily a low-resistance ground.
In my tests over the years, a 3/4-inch copper pipe driven five feet or deeper into the soil typically measures between 50-150 ohms of RF resistance on 160-meters. (DC or low frequency AC measurements will NEVER give the correct earth resistance for RF, and they certainly can not tell us ground conductivity.) Unless you have exceptionally poor soil, going deeper than five feet will not reduce RF resistance on frequencies above 1.8 MHz. Skin effect limits the depth of RF current in the soil, so the extra rod depth does nothing. Lower resistance values (about 55 ohms) were obtained in a wet marshy area of NW Ohio, with a very rich black acidic sandy loam soil. The higher resistance were obtained in rocky clay soil typical of the Atlanta, Georgia area.
My present location has rolling pastures and wet clay soils, providing under 100-ohms of RF resistance at 1.8MHz with a five-foot rod.
The general guideline I follow is to use at least two five-foot copper rods (I use 3/4" copper spaced 5 feet apart). If I can not get full depth, or if the soil is particularly poor, I add a few 30-60 foot buried radials. The idea is to obtain a reasonably stable ground, so termination does not change.
If you are unsure if you Beverage's ground is adequate, measure the impedance of the beverage with an antenna analyzer with your operating ground systems. Note the reading. Add two temporary radials 1/4 wl long suspended above earth at right angles to the Beverage, and re-measure the impedance. (It is OK to have them there at right angles to the antenna and not have them connected, and them connect them while taking readings.)
You can measure the impedance on the low-Z side of a good transformer. Under almost any condition, the wires would have 100 ohms or less impedance. If you see a very noticeable change in impedance, you probably should consider improving the ground system. Impedance changes of 15% (or larger) indicate a potential ground stability problem, because the ground resistance would be nearly 100 ohms. This test should be done when the ground is dry, or any time you think you might be having a ground problem.
Always remember to keep the shield of the cable isolated from the Beverage ground! Never use un-un or autotransformers.

Length

For length considerations, see the directivity factor text. It is not necessary, nor does it do any good, to go beyond 1-1/2 or 2 WL. By the time the antenna is that long, current is so low any addition length makes the pattern worse. I limit my 160-meter antennas to 800-feet, and use multiple antennas when a sharper pattern is required.
Directivity can actually decrease if a longwire-type array is made too long. This is true with Rhombics and Vee Beams, and it is also true with Beverages.

Zigzagging Wire

While a nice clear straight wire looks great, it does more to make us feel better than hear better! Minor ups and downs in height or dips or valleys don't really seem to have any noticeable impact.
Although it probably is a good idea to keep the wire as straight as possible, it is the overall direction and length that is most important because each small area contributes on a similar small portion to the overall directivity and signal reception.
Tom, W8JI
If You would like to know more about author nor You are interested in more informations about receiving antennas and not only about that then visit web pages of Tom, W8JI please on http://www.w8ji.com/
73, Petr OK1RP
 

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