SORAK

Special Operations Radio Antenna Kit OE-452/PRC

NSN 5985-01-279-7942

Brooke Clarke 2005 - 2016



2 Bags

Background
Supported Radios
Antenna Configurations
    Beads
    Stock Configurations
    Alternate Configurations
Test Sets
    TS-4350 Power - Tx VSWR
    TS-4351 Noise Bridge - Rx VSWR
Manuals
Description
Operation
Broken 6' Mast Cap
Links

Background

There have been a number of antenna kits designed for easy portability and this may be one of the latest.  During the Vietnam conflict antenna kits were developed for the VHF low band squad radios like the OE-303 half rhombic and for H.F. NVIS use like the AS-2259.  The H.F. antenna kit for the GRC-19, the GRA-50, was also used.  This SORAK antenna seems to combine both bands into a single antenna kit.

I had seen a web reference to a SORAK antenna and in that reference it talked about buried antennas.  This antenna does not support any on the ground or buried configurations based on the manual fragment I have.

In all the SORAK configurations the near end of the antenna is connected to the radio using a BALUN (BNC 50:430 Ohm) and the far end(s) of the antenna wire(s) are terminated with 430 Ohms.  This probably has been done to support the frequency hopping modes now used on both HF and VHF military radios.

It's interesting that the termination impedance is higher than the free space impedance of 377 Ohms (PI*120).  If the antenna wire is long enough for a voltage maximum to appear then very little energy would be left that can get to the terminations.

Supported Radios

Radio
Frequency (MHz)
PRC-70 2 - 75.975
PRC-74, 74A 2-11.999
PRC-74B, -74C
2-17.999
PRC-77 30.00 to 75.95
PRC-104() 2 - 29.999
PRC-119 SINCGARS 30 to 87.975
PRC-132 1.60 - 49.99
In PRC-132 TM 11-5820-1102-12 in addition to showing the SORAK antenna is also shows the OA-8990/G DMDG, G-76/U hand crank generator, KY-116/U leg key, H-250 handset, H-251 headset

Antenna Configurations

The names used in the SORAK TM are different from the names used by hams for the same configurations.  The length naming convention is also very counter intuitive, that's to say the manual leaves a lot to be desired in terms of clarity.

There are two ways an H.F.  radio can match into an antenna.  One is when the antenna system is near 50 Ohms in impedance and the radio sends out the R.F. without using any antenna coupler.  The other is when the antenna has a VSWR typically greater than 3 and the radio's internal antenna coupler is used to get a good match.  When using the 50 Ohm radio output any reasonable length of coax can be used with only a slight penalty in coax loss.  When using the radio's internal antenna coupler and coax between the radio and antenna degrades the ability of the coupler to match the antenna. 

Since the SORAK BALUN is designed for a 430 Ohm load it appears as if the SORAK user is not supposed to use resonate antennas, only the wide band configurations shown in the manual.  In all the configurations the near end of the antenna is connected to the radio using a BALUN and the far ends of the antenna wire(s) are terminated.

Beads

There are beads crimped to the antenna wire so that you can stop at the desired lengths.  Instead of using three different bead configurations they depend on knowing which beads have already been passed.

Beads
Length
stop at first bead
117'
pass one bead
stop at 2 beads
175'
pass one bead
pass two beads
stop at one bead
234'

Stock Configurations


175 Foot Bent Longwire, 234' total
(Vertical Half Rhombic)
30 - 88 MHz

SORAK OE-452 175 Foot Bent Longwire (Vertical
                  Half Rhombic) 30 - 88 MHz

In this configuration the near end of the antenna wire is connected to the radio using the BALUN and the far end is terminated with 430 Ohms.  This configuration is primarily intended for use in the VHF low band, very much like the OE-303.


117 Foot Sloping Dipole 234' total
(Inverted "V")
2 - 10 MHz

SORAK OE-452 117 Foot Sloping Dipole 234' total
                  (Inverted "V") 2 - 10 MHz
This would almost be called an inverted "V" in ham radio literature.  I say "almost" because in this case the antenna is not resonate and the wire ends are terminated, whereas for a ham radio antenna the ends would be left open.  Note that the common formula for calculating a wire dipole is total length (ft.) = 468 / Freq (MHz).  Using this formula you would calculate that for 2 MHz the length should be 234 feet.  Since the wire ends are terminated the length is not really critical except that it needs to be greater or equal to a half wavelength at the frequency of operation in order to get useful radiation.

At the lower H.F. frequencies the antenna height for 1/4 wave varies from 123' at 2 MHz to 8 feet at 30 MHz.  So for the low H.F. frequencies that antenna is close to the ground and operating in the NVIS (Near Vertical Incidence Skywave) mode where the signal is sent almost straight up.  This is similar to the AS-2259 except the SORAK mast is much taller and the wires are much longer so it should work much better than the AS-2259.

In the 2 to 10 MHz range there's good radiation straight up, good for NVIS operation.  Note the diagram in the manual says that the gain at 2 MHz is about -14 dB and at 10 MHz about -10 dB numbers not uncommon for NVIS work.

117 Foot Sloping Vee
Horizontal Half Rhombic)
4 - 12 MHz

SORAK OE-452 117 Foot Sloping Vee Horizontal Half
                  Rhombic) 4 - 12 MHz
The plan view of this antenna looks like a standard rhombic that has had the two far legs removed and the terminations added to the existing legs.  Intended for use in the 4 to 12 MHz frequency range.

It looks like when they made the manual the figure for the 234' Sloping Vee was cut and pasted into this section, i.s. both figures look the same.  I think this one should show a wire length of 117' instead of the 234' shown.

So there's come confusion between the two versions of a sloping Vee (hald rhombic).

234 Foot Sloping Vee
(2 ea 234' wires 45 included angle,
Horizontal Half Rhombic)
8 - 24 MHz

SORAK OE-452 234 Foot Sloping Vee (2 ea 234'
                  wires 45 included angle, Horizontal Half Rhombic) 8 -
                  24 MHz
The plan view of this antenna looks like a standard rhombic that has had the two far legs removed and the terminations added to the existing legs.  Intended for use in the 8 to 24 MHz frequency range.  Single bead on antenna wire.

468 Foot Bent Longwire
(Vertical Half Rhombic)
8 - 20 MHz

SORAK OE-452 468 Foot Bent Longwire (Vertical
                  Half Rhombic) 8 - 20 MHz
This is what's more commonly called a Half Rhombic similar to the OE-303, but in this case made longer to work in the HF frequency range.  The radio is connected to one end that's supported on a 6' mast using a 6' coax, the 22' mast is holding up the center and a termination is connected to the far end also supported by a 6' mast.  This configuration should provide the longest range (1500 to 2500 miles) operation over the 2 to 20 MHz range.  It's a directional antenna with the main lobe radiating off the antenna toward the termination.

In the 8 to 20 MHz H.F. frequency range the pattern is in the 10 to 20 degree elevation range, but at 30 MHz there are many many lobes in the 50 to 90 degree range where they are of no value.  The peak gain may be as high as 8 dB.

Alternate Configurations

Wire Length

There are two antenna wire reels and each of them can be unrolled to three lengths: 117', 175' or 234' if only the existing beads are going to be used.
There are always two legs because of guy tension and/or electrical symmetry.
But there's no requirement of equal lengths, so each leg can be either 117', 175' or 234' independently of the length of the other leg.  This might improve the match and or pattern for some frequencies

Plan Arrangement

There are two physical arrangements of the antenna legs.  They are either in a straight line or in a Vee.  The straight line antennas are called dipoles, or long wires and the others "Vee".

Connections

Stock Balun & 2 Terminations
There are always three electrical nodes: two terminations and one feed balun.  The blaun can be installed either at an end or at the center.
It may not make sense to feed a "Vee" from an end even though it's possible and might make for a interesting experiment.

Dipole (Balun & 2 wires)
This would require setting the length of the antenna wires to make a resonate dipole antenna.  This can be done using the supplied white noise bridge and knowing the equation: dipole end to end length in feet = 468 / frequency in MHz. to estimate the length and then pacing this off on the ground to make a first try.  Then erecting the dipole and checking with the noise bridge.  The percent error in resonate frequency and be used as a percent correction to the length to tune the antenna.

Loop (Balun & no terminations)
A new connection type would be to connect the two antenna wires together and connect the balun to the other two wire ends forming a loop.
An example of that would be to install the balun at the top of the 22' mast, run each wire down to a 6' mast and then run the wire along the ground and join the two ends at the base of the 22' mast.
Maximum length inverted delta loop with top feed
22' mast - 6' mast = 16' triangle height, from the Pythagorean theorem (Wiki)
let D = length of horizontal wire on ground from 22' mast to 6' mast.
then the length of the wire from the ground to the top of the 6' mast is 6;, then to top of 22' mast is 234'-6'-D or 228-D

[228-D)^2] = [51984 - 456D + D*D] = 16^2 + D^2 = 256 + D*D, or 51984 - 456*D = 256, or 51728 = 456*D, D=113.4'
sloping wire = 234 - 6 - 113.4 = 114.6'
Dipole
This is not an option because there's is no dipole center connection method supplied in the SORAK kit of parts.  But it could be easily added from the GRA-50.  Adding the frequency calibrated tape measure from the GRA-50 would allow the SORAK to work like a GRA-50.

Permutations & Combinations

All of the antenna configurations above could be modified by changing the wire length.  That also might improve the pattern/gain/match for some frequencies.

One example the 234' inverted V that now works down to 2 Mhz could be extended to 1 MHz by making each leg 234' long (468' total).

Test Sets


TS-4350

TS-4350
                drawing


The TS-4350 is a power meter that is used with a transmitter to measure forward and reverse power allowing determination of VSWR.

This is very similar to the PRM-34 Test Set or  better the
URM-182A TS-3754/U VHF Low Band Power Meter

"...it is marked 2-88MHZ, it has a internal load feature, forward and reflected power setting and will read 0 to 50 watts".  K1HF

TS-4351

TS-4351
                drawing


TS-4351 is a noise bridge that can be used to check antenna VSWR at the frequency set on the receiver
TS-4351 antenna noise bridge

Off turns off the internal 9 volt battery.
Pressing button on side turns off noise.
Probably usable for HF and VHF low band.

I got this one from Mike Murphy.
TS-4351 is a noise bridge Spectrum
                Plot
Plomar Engineers used to make a "R-X Noise Bridge" that covers 1 to 100 MHz, but it's now obsolete.  manual + added info at Bama, they show up on eBay.

Wiki "Antanna Analyzer" is about the noise bridge.

Googling for "Antenna Noise Bridge" will bring up ready made units and kits.
Far Circuits has the PCB for the QST Dec 1987 Noise Bridge for under $10.

AEA -
Autek Research
MFJ Enterprises
Palstar
New Jersey QRP Club - Rainbow Antenna Analyzer 

Manuals

TM 11-5985-391-12 Special Operations Antenna Kit (SORAK), Antenna Group OE-452/PRC, NSN 5985-01-279-7942
This manual dated 1 Jan. 1991 has a number of errors.  Most notable is that the wrong number of beads is shown in the diagrams to indicate how much wire should be unreeled to get the proper length.  One bead is shown for almost all lengths.  Does not mention the test sets.
TM 11-5985-391-20P Parts

Description

The Kit was made by CAGE code 1L397 which is GKS Inc., 25670 1st St., Westlake, OH 44145 - 1493.
Their CAGE listed phone number no longer is in service.

The SORAK is packaged in two field bags made of rip stop nylon.  Velcro is used extensively and plastic snap ends on the straps.

Small Bag

The smaller one is a roll about 17" long and 8" in diameter weighing 14 pounds.
Marked: 80083-A3158103, MFR 1L397
Has 8 Velcro pockets:

Large Bag

The large bag is 5" by 8" and 30" long and weighs 9.5 pounds.  It holds the extendable 22' mast and the two 6' extendable masts.

The two 6' masts have their 3 guys and stakes attached.

The 22' mast is 28" long when collapsed and the O.D. of the bottom tube is 3.84".  The top mast is a rod 0.80" in diameter.  All mast parts are made of what may be fiberglass reinforced plastic.  The locking method between sections is similar to a bayonet connector.  The 22" mast is designed to be erected by extending it straight up and only lifting the guy ropes and a halyard instead of the antenna wire.  This way the weight being lifted is minimized.  Once the mast is erect and guyed the halyard is used to raise the antenna.

Electrical

BALUNBALUN

The BALUN is matched to the 430 Ohm termination.  More than 20 dB return loss from 2 to 76 Mhz when one of the 430 Ohm terminations is connected to the balun.

This explains why there is no mention of adjusting the antenna element lengths to get a resonate antenna.  If that was done the antenna would look like 72 Ohms real at the BALUN terminals and would be a 6:1 mismatch.




TerminationTermination

Although it's 9 inches long the internal Carborundum Corp 430 Ohm resistor is about 4 inches long and maybe 5/16" diameter.

Operation

So far (Jan 2005) I only have a manual copy with only a small number of pages and so can only speculate on actual uses.  From the manual fragment it looks like there are only the 5 configurations supported.  Not like the GRA-50 where the antenna length is adjusted by how much wire is unwound from the reels.  But I don't see any reason why you should not use tuned antennas, unless it relates to the BALUN impedance matching ratio.

Broken 6' Mast Cap

Broken CapOne of the 6 foot masts has had it's cap decapitated.   I don't know when this happened, but suspect that it was caused by the heavy 22 foot mast banging against the shaft part of the 6' mast while the 6' mast cap was on the ground or floor.  Putting the light weight 6' masts right next to the heavy 22' mast in the same carry bag doesn't look like a good thing to to.

There's a hole in the cap and in the mast section that's a little under " in diameter so the fix is to use J-B Weld on a " wood dowel and epoxy them back together.  The hole in the cap is a little more than " in deep.

You can see the 3 parachute guy lines looped through holes in the cap.

I think is was broken while in the carry bag.  You could cause that type of failure by setting the mast on the ground and giving a karate chop to the mast just below the top cap.  If the mast was extended and you did the same thing it would be much more difficult to break off the cap since the mast would tend to flex which it can't do when collapsed.  When all three masts are in the carry bag and someone drops the bag or the bag is standing up and falls over the 22 foot mast would deliver exactly the type of blow that would decapitate the six foot mast cap.

Broken Mast
On the photo at left I have added a mark showing about where the hollow part is located.  It's just over an inch deep.

So the dowel needs to be about 1" in long.

After the J-B Weld setup overnight the mast is probably stronger than a stock mast because of the reinforcing of the " dowel.

There probably is a way to make the mast carry bag more protective of the masts it's holding.



Links

 
Cebik Amateur Radio - Some Notes on NVIS Cloud BurnersNotes on the Terminated Wide-Band "Folded Dipole" -
CSA Wireless - the Fanlite & Longshot antennas look very similar to the SORAK configurations
Ten-Tec - Model 3402 and 3403 -1.8-54 or 3.5-54 MHz. looks just like the 117' sloping Vee.


Brooke's Products for Sale, Military Information, Antennas, Home 

[an error occurred while processing this directive] page created 13 Jan 2005.