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TECH NOTES—GROUND RADIAL SYSTEMS

In almost every case the efficiency of a vertical antenna will be

greater if radial wires are used to improve ground conductivity as

in figure 2.  It’s important to note that there’s no point in cutting

radials to any particular length when ground mounting because

the earth will detune them anyway.  All you want to do is make

the surface of the earth around the antenna more conductive than

it is ordinarily.

If you can’t copper-plate the backyard, the best approach is to

run out as many radials as possible, each as long as possible

around the antenna in all directions.  Radials may be left on top of the ground

however they should be buried for the sake of pedestrians and lawnmowers.

How long should radials be?  A good rule is no shorter than the antenna is tall

because 50% of your losses will occur in the first 1/4 

8

 out from the antenna.  If

you have more than a dozen radials, they must be longer to get the most out of

them which is why the FCC specifies 113 wires each .4 

8

 for AM broadcast

stations—the equivalent of a zero-loss ground plane.  Obviously, for most ham

work this would be overkill.

In some cases wire mesh (i.e. chicken wire) may be used as a substitute for radial

wires and/or a ground connection, the mesh or screen acting as one plate of a

capacitor to provide coupling to the earth beneath the antenna.

It should be noted that a ground rod is useful only as a d.c. ground or as a tie point

for radials.  It does little or nothing to reduce ground losses at r.f. regardless of

how far it goes into the ground.

Bare wire, insulated, any gauge, it doesn’t matter.  The current coming back along

any one wire won’t amount to that much.

EFFICIENCY

The importance of reducing losses in the ground system can be seen from an

examination of a vertical antenna's feedpoint impedance which at resonance

consists of three components: antenna radiation resistance; conductor loss

resistance; and earth loss resistance.  An unloaded quarter-wave vertical antenna

has a radiation resistance of about 35 ohms with negligible ohmic or conductor

loss, but ground loss resistance may be very great if no measures are taken to

reduce it, and in some cases ground loss R may even exceed the antenna radiation

resistance.  These three components may be added together to arrive at the

feedpoint 

impedance

 of a 

resonant

 (no reactance) antenna.  For the sake of

illustration, assume that the ground loss beneath a quarter wavelength vertical

antenna is 15 ohms, that conductor loss resistance is zero, and that the radiation

Содержание HF9V

Страница 1: ...GROUND IT IS ALSO RECOMMENDED THAT ALL STATION EQUIPMENT BE DISCONNECTED FROM THE POWER MAINS BEFORE CONNECTING THE FEEDLINE TO THE ANTENNA PLEASE CONSULT THE A R R L HANDBOOK OR OTHER REFERENCE MANU...

Страница 2: ...he end of the tube Pass a 8 x 1 1 2 screw through both parts and secure with a lock washer and hex nut 4 From the center of the insulator measure downward to a point that is 13 in 33 cm along tube B a...

Страница 3: ...t 40 meter D2 and install on the smaller 67 pF capacitor as above 12 Install the above assembly onto the 10 screw protruding from the tab of the center clamp on the coil assembly 80 40 meter C Align c...

Страница 4: ...et O1 around the fourth turn of the 40 meter coil on coil assembly 80 40 meter C Secure with a 8 x 3 4 screw lock washer and hex nut 24 Attach strip 17 meter X to the bolt that fastens the coil to the...

Страница 5: ...so the top edge is 33 1 4 in 84 5 cm from the upper end of the tube 38 Pass the free end of the stranded wire from wire clamp 0 625 6 M w wire T through the small hole in wire clamp 0 750 6 M w insula...

Страница 6: ...he uncapped end of tube J into the slotted end of tube I until only 25 in 63 5 cm extends and secure with compression clamp small adjustable NOTE In the following steps the antenna will be assembled a...

Страница 7: ...and may be as much as 100 kHz at the high end of the band depending on the efficiency of the ground system used greater bandwidth being associated with lossy ground systems It should be remembered th...

Страница 8: ...e and lowest VSWR NOTE To minimize interference to other stations and to avoid erroneous reading use only enough power to produce full scale deflection of the meter in the forward or r f out position...

Страница 9: ...re clamp 0 875 15 M w insulator K A change of 2 in 5 1 cm will shift the VSWR curve approximately 300 kHz 6 Check VSWR on 10 meters To raise the resonant frequency loosen the small hose clamp over the...

Страница 10: ...equency in all installations In the preceding steps it has been assumed that the antenna has been installed in a more or less clear spot away from other vertical conductors such as TV antenna feedline...

Страница 11: ...nnas an electrical 1 4 8 section of 75 ohm coax is used as a geometric mean transformer to match the 100 odd S of feedpoint impedance on that band to a 50 S main transmission line of any convenient le...

Страница 12: ...ylene rope may be used but because some grades tend to deteriorate fairly rapidly periodic inspection should be made A single set of guys placed just above the 30 meter circuit will contribute greatly...

Страница 13: ...ter L Bracket P 00249SZV Coil Capacitor Assembly 30 Meter Q 00137SZV Coil Q Base Matching R 290 08 Coax 75 Ohm Matching S 00228GZV Connector PL 258 T 00331RZV Wire Clamp 5 8 6 M W Wire U 00332RZV Wire...

Страница 14: ...14 Feedline Detail PICTORIAL...

Страница 15: ...G AND WEAKEN THE INSTALLATION If U bolts must be used place a larger diameter metal such as the MPS Mounting Post Sleeve over tube w insulator A Similar precautions should be observed when using TV st...

Страница 16: ...cken wire may be used as a substitute for radial wires and or a ground connection the mesh or screen acting as one plate of a capacitor to provide coupling to the earth beneath the antenna It should b...

Страница 17: ...verticals for reasonable efficiency If the ground loss of resistance of 15 ohms from the preceding example could be reduced to zero ohms it is easy to show that the efficiency of our one eighth wavele...

Страница 18: ...that band It s important to note that cutting each conductor of rotator cable to a specific frequency will not work unless you separate it angling each conductor away for most of its length because t...

Страница 19: ...a supporting metal structure for antenna currents that might otherwise flow on the structure and thus turn the vertical antenna system into a vertical long wire with unwanted high angle radiation OTHE...

Страница 20: ...but it is generally considered that the best possible location for a mobile antenna is in the middle of the roof of the vehicle i e at the center of the vehicle s ground plane and at a point where the...

Страница 21: ...building Steel TV mast sections are readily available in lengths of ten feet and the mounting posts of Butternut HF verticals will slide into those which have an outside diameter of 1 1 4 inches and...

Страница 22: ...tion 6 in or less Symptom High SWR on one band when antenna is roof mounted Radials are in place but antenna will just not tune Look for Radials of wrong length or running close to metal rain gutters...

Страница 23: ...ged or damaged Build up of salt or pollution deposits on insulators and capacitors Radial system corroded or rotted away Owner must do routine maintenance at intervals according to local conditions Sy...

Страница 24: ...ure of an antenna to wind or weather c from improper assembly installation or use of an antenna or d from failure periodically to inspect and maintain an antenna and its installation The Customer is r...

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