AN/GRA-50 Antenna

Background

Made for use with the vehicle mounted AN/GRC-19 Radio set consisting of the T-195 Transmitter and R-392 Receiver. 1.5 to 20 MHz coverage at 100 Watts.  This antenna was intended for field use, not base station.  It would be deployed by throwing the weighted end of the halyard over a tree branch.  Since the halyard is 75 feet long the highest the antenna can be is about 40 feet up.  For operation at 1.8 MHz (160 meters) the antenna should be up at least 1/4 wave length ( 40 meters or about 120 feet) for good horizontal propagation.  So for frequencies below about 6 Mhz the antenna acts like a Near Vertical Incidence Skywave (NVIS) antenna and sends most of it's energy straight up.  This provides good coverage out to a few hundred miles.  The manual mentioned that in a jungle environment a 15 foot vertical antenna looses a lot of it's signal to the trees and the GRA-50 will do much better.

Construction

The two dipole elements are on reels so that the length of the doublet can be adjusted to match the frequency of operation anywhere in the 1.5 to 20 MHz range of the T-195 transmitter.  But it could be used at higher frequencies.

Installing Wire on Reeling Machines

A new GRA-50 comes with the antenna wire separate from the Reeling Machines.  To install the wire:
In a location where you can work with 156 feet of wire tie off the inner end of the wire and carefully remove one turn at a time as you work your way to the other end.  Attach the end to the reel in such a way that when the wire is pulled it will tend to tighten in the slot rather than come loose.  Connect one end of the wire to the reel by using a Wing Nut installed from inside to the Large hole.  The washer will fit on the wing nut side of the hole.

Selecting Wire Length

The tape measure can be used or the formula Length of each antenna wire in feet = 234 / (Freq in MHz)
This is equivalent to the overall dipole length in feet = 468 / (Freq in MHz)

The amateur bands are harmonically related so if the antenna is setup for 160 meters it should theoretically also will work on 40 and 10 meters but not 80 and 20 meters.  i.e. it works where each wire is an odd multiple of a quarter wavelength.  Because of ground proximity at the lower frequencies it's not clear how the antenna impedance behaves with frequency, something to actually measure.  Stay tuned (20 Oct 2002).

Using the Wire Clamp Wing Nut on the Reeling Machine

Center Insulator

The Center Insulator has holes to allow hanging it but the standard GRA-50 kit does not include a Halyard for that purpose.

With the type N connector pointing down and the wing nuts visible the left wing nut is connected to the shield and the right wing nut to the center conductor, if you want to make a vertical antenna.

Parts List for GRA-50 FSN 5985-892-0758

 

Photo	Description	Nomenclature	Qty	FSN
	Technical Manual		1	TM 5820-467-15
	Bag	BG-175	1	6115-498-3973
	75' RG-58 Cable Assy 2 ea N(m) UG-536/U	CG-678/U	1	5995-823-2176
	Center Insulator with N(m) connector & cap	IL-4/GRA-4	1	5970-405-8223
	1/8" dia x 75' Halyard with 8 oz lead weight	MX-2706/G	2	5985-893-1438
	Cable Reeling Machine	RC-432/G	2	5895-896-3179
	160' Antenna Wire	CX-7303/G	2	5985-757-2130
WANTED	Measuring Tape	Arkay # 859-501	1	5210-897-6077

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