Antennas

Brooke Clarke, N6GCE
General Information
VLF Antennas
HF Antennas
    Long Range
    Medium Range
    Ground Conductivity
VHF Low Band Antennas
    Mast Mounted
    Mobile Mounts
    Hand Held
    Man Pack
    Field
    Body
VHF High Band Antennas
UHF Antennas
Microwave
Direction Finding
    Receiving Loops
    Other DF
Antenna Theory
Antenna Related
    Supports 
    Pre-Amplifiers
    Antenna Tuners
    Antenna Couplers
    Antenna multicouplers
    Duplexer
    Time Delay Beam Steering
     Counterpoise
Pattern
Patents
Links

General Information

The antenna used for either receiving or transmitting is a vital element in the communication link.  In most cases the antenna has a great deal to do with how well the receiver or transmitter works.

Antennas are reciprocal, meaning that an antenna designed for transmitting will work for receiving.  A passive receiving antenna will also work for transmitting.  BUT the goals are different for reception and transmission.  In general a receiving antenna should provide a good signal to noise ratio.  In general a transmitting antenna should radiate energy as efficiently as possible in the direction of the receiver.  A loop may make a great receiving antenna for a broadcast band radio but would be terrible as a transmitting antenna.  Transmitting into an active receive antenna will let the smoke out of it.

VLF Antennas

By far the best performing low frequency antenna I've used is a long wire (say 100 feet) in series with a variable inductor (Miller 3 mH).

McKay Dymec DA-100 Active Whip - Board Layout & Circuit Diagram - through 30 MHz coverage
McKay Dymec DA7 Loop
AMRAD - Active Low Frequency Antenna has coverage to 30 MHz
AS-2108/ARN-89 crossed active loops for chopper radio direction finding
Light Weight Beacon - a 50 foot tall antenna system that fits in a back back along with a transmitter
Home Brew Loopstick for WWVB - work in progress, not working yet

HF Antennas (2 to 30 MHz)

The key idea about HF radio is the propagation by reflection from the ionosphere.  Frequencies below 2 MHz or above 30 MHz can not reflect off the ionosphere.
Although you can use HF to cover short distances without the ionosphere, called ground wave propagation (Wiki) which works well for lower frequencies, it is very poor method at HF.
Successful HF propagation depends on reflecting the signal from the ionosphere.  There are two common methods of doing that.

Long Range

In this mode a signal can goo all the way around the Earth (and sometimes has been seen to make multiple revolutions) so can cover great distances.
See the Very Basic Antenna Information at the top of my HF Propagation web page to help understand propagation modes and which antenna type supports them.

A common antenna would be a half wave dipole mounted 1/2 wave above a perfectly conducting surface.  If the dirt under the dipole is not very conductive then it could be lower and work well. The idea is that the main lobe is broadside to the wire and covers horizon to horizon including straight up.  Note for a perfect antenna there are nulls off each end where there's no propagation.  This is what's called a DX antenna.

Medium Range

I'm using the word medium to differentiate between line of sight which I would call short range.  Near Vertical Incidence Skywave (Wiki NVIS) is a term coined during the Vietnam ear for the idea of aiming an HF signal directly overhead so that when it reflects from the ionosphere if comes back down to Earth an illuminates a circle that's 500km (300 miles) in radius. But this mode of operation has been used long before Vietnam, it just didn't have that name.

A common antenna would be a half wave dipole mounted 1/4 wave above a perfectly conducting surface.  If the dirt under the dipole is not very conductive then it could be lower and work well.  The idea is that the main lobe goes straight up and there are nulls at the horizon in all directions.  This provides higher gain than the DX dipole described above because the power is more confined in where it can go.  (PS.  antenna gain and antenna pattern are a function of each other.   For example you can not have an omni directional antenna with high gain).

If the antenna is lowered there are two effects:
1) the received noise level is decreased, and
2) the transmitted signal level is decreased.
So lowering below 1/4 improves reception, but degrades transmission.

See my AS-2259 Antenna web page for more on NVIS

Ground Conductivity

This has a stronger effect on NVIS antenna than it does on DX antennas, but still is important for DX.  Note that in the days prior to satellite communications the long range HF radio stations were located very close to the edge of salt water where the ground conductivity was low.  The 1/4 wave vertical antenna towers used for AM radio transmission typically have 120 radial wires so that the ground conductivity is controlled.

In English we have different meanings for "ground" (Wiki).  One meaning is dirt another meaning is an electrical terminal.  In the case of the "ground" under an NVIS antenna the operative definition is electrical terminal (not dirt).  So it's not a simple matter to determine how high above ground you are mounting the antenna.  In the desert where sand is an electrical insulator a buried NVIS antenna may work great but an NVIS antenna a foot or two above fertile agricultural soil may not work at all.  I suspect there are simple methods of measuring the resistance of the soil maybe just using a DMM in the Ohms mode or using a 4-point probe (Wiki: 4-Point Probe, Volume Resistivity)made from a 2x4 and some long nails.  But there's probably a frequency dependance of the soil resistance making the measurement much more complex.

Dipoles & Wires

The B&W Broadband Folded Dipole Antennas (ASW-90) U.S. Patent 4423423  is an excellent receiving antenna for broad band signals like chirp sounders.  This design is also called a T2FD.  The efficiency for transmission goes down rapidly below 40 meters.  Works far better than any active whip or loop.
OE-452 - NSN 5985-01-279-7942, Special Operations Radio Antenna Kit  SORAK
AN/GRA-50 Antenna, HF NVIS Dipole w/reels at the ends
AS-2360 Loop Antenna Parts List & Radio Receiving Set AN/TRQ-23 Antenna Group OE-4/GR AS-2360 Series Loop Antennas
AS-2259 or AS-2268 (Collins 637K) NVIS tatical antenna
GRA-50 1.5 to 20 Mhz Reel dipole where the reels are at the far ends, not the center.
AS-1321/PRC-47 Antenna 45 foot long wire
AT-984/G 150 foot wire
Eyring Low Profile Antenna 301A (ELPA 301A)
Hy-Gain HA-4000, 18TD, Collins TD-1, 637T-2 - a dipole made with two steel tape measures as the center part.  You pull out the tapes, marked in the metric system and you're good to go
Collins
                637T-2
637T-2 has two reels with antenna wire on each.  Center support hole on top and type N(f) connector on the bottom.  Used to quickly and easily make a dipole for NVIS ops.

SORAK - HF & VHF low band configurations
TCI 651T crossed delta loops

Whips

AS-1320/PRC-47 Antenna 15 foot whip
GRC-106 15 foot whip -photo - in CW-206 bag - Fair was selling these as a GRA-50, but they are for some transmitting application because of the insulating sleeve for the lower whip (ms-11x whip sections.  Not the receive only antenna for the R-442.)
AT-271 Fishing Pole 3 meter
AT-1011 Shakespeare 120 HF antenna system 12, 16 or 32 foot whip
Eyring Low Profile Antenna 301A (ELPA 301A) can be buried 1 foot deep
TCI 651T -HF portable antenna, single mast
M-442 Adapter
M-442 Ant Adapt


Accepts AB-21 or MS-116 tubular antenna elements and has common 3/8-24 male thread to fit the PRC-104, AB-591, etc.

got this one from Fair Radio
Tape Whip Adapter
TWA

Accepts 5/16-24 male thread on mil antennas and screws into 3/8-24 ham, CB, commercial antenna bases.  The 5/16-24 female thread is deep enough to allow the AB-591 antenna base to be used.

Ship's Goniometer NUS-883 - photo - Adcock Antenna (Wiki) array for mounting on ship's mast
PRD-1 Loop
PRD-1 HF DF
                  Receiver



The PRD-1 receiver uses a single turn diamond shaped loop made of tubing.


VHF Low Band Antennas

30 - 90 MHz

Line Of Sight

The range between a standing manpack and a manpack lying on the ground is about 2.7 miles.  Between two standing manpacks it's about 5.4 miles.

Antenna elevation is the key factor for this band.  Power is not that important.  Most radios have a few watts of output power and that's all that's needed for normal links.

Mast Mounted

OE-303 1/2 Rhombic

OE-254 Bi-Conical

RC-292 1/4 Wave Vertical

Create Discone - this makes a great scanner receive antenna when a Radio Shack 15-1170 amplifier is used.
Shown disassembled with roof mount tripod.
Maybe 20 MHz to 1 GHz?
Create Discone antenna

Create Log Periodic
Maybe 20 MHz to 1 GHz?
Create Log Periodic
            antenna


SORAK - HF & VHF low band configurations

Mobile Mounts

AS-1729 Remote tuned vertical dipole (MX-6707)

Hand Held

AT-784/PRC directional Loop
Rubber Duck - used on PRC-68 family of Squad Radios (Note: this antenna has 30 dB less radiated power than the AT-892 on a PRC-68!, that's NOT a typo)
AT-892 Tape - used on PRC-68 family of Squad Radios
68AA Antenna Adapter - fits the special PRC-68 family of hand held squad radios and has a standard BNC(f) connector plus a DC return to the more modern radios in the family will correctly know an external 50 Ohm antenna is connected.

Man Pack Radio Mounted

AT-271 3 meter fishing pole used with the AB-591 spring base
AS-2109 Telescoping that can act as either a 1 meter or 3 meter antenna for the PRC-25, PRC-77 etc.
Shortened Antennas for Portables by Dennis Starks

Field

AT-984/G Fishing Reel 150 foot wire used for HF and VHF antennas

Body

soldieer's body mounted antenna


Soldiers carrying an antenna are high on the enemies target list so they typically do not unfold the antenna which means the radio barley works if at all.  So I've read that an antenna was developed to be wrapped on the body of the soldier carrying a radio so as to not have an easily visible antenna.  I think that's what this is, but there are no markings.  It has a TNC-m connector so not sure which radio it fits, maybe it is not a VHF low band antenna, TBD.
The 50? Ohm line from the connector to the antenna is about 9" long.  The antenna is about 100 inches (2.5 meters) long.  The lanyard is about 19" long.

If you know about it please contact me.

None of the below seem to match this antenna.  Maybe it was a prototype that didn't work?
The COMWIN Antenna Project (slides).
7471258 Coaxial cable having high radiation efficiency,  Hrl Laboratories, Llc, Dec 30, 2008, -
7956818
Leaky coaxial cable with high radiation efficiency, Hrl Laboratories, Llc, Jun 7, 2011, -     
8059045
Antenna having an impedance matching section for integration into apparel, Hrl Laboratories, Llc, Nov 15, 2011, -
9209514 Body-worn antenna, Motorola Solutions, Inc.Dec 8, 2015, - aimed at undercover police work - concern for RF exposure     

VHF High Band Antennas

108 - 170 MHz

Eggbeater - has a circularly polarized pattern going up which is good for working satellites and has vertical polarization in the direction of the horizon which is good for working most VHF stations. See Brooke's Military Information/Equipment  for more on this one.
AS-3588/GRC-206 Combined VHF-AM & UHF-AM Antenna for GRC-206 (PRC-104 page, MT-6250 page)

FM Radio - FM stations can use horizontal polarization to reach receivers in stationary locations with indoor or outdoor antennas.  They can also use vertical polarization to reach automobile whip antennas.  Most stations divide their power between the two depending on their target audience, commuters, at work, commercial use, etc.  A vertical antenna may work better at your home if the station you want to hear is targeting commuters.

Radio Shack 20-176 Scanner 1/4 wave (this model is obsolete, replaced by the Model #: 20-043)

Radio Shack 20-176
          Scanner 1/4 wave antenna

UHF Antennas

This is the military 200 (or 225) to 400 MHz band used by aircraft in the AM mode, for Line Of Sight (LOS) communications by ground troops, and used for linking to satellites.
AS-390A/SRC has ground radials for pole mounting and painted navy grey, like for a ship or control tower.
AS-1405/PRC-41 Log Periodic
AT-803/VR has same antenna element as AS-390A/SRC but is intended for mounting to a metal shelter roof (OD paint).  mentioned in FM 24-24 as part of the VRC-24 & TRC-68 systems.
Satcom antennas
Trivec-Avant AV 2095 UHF Satcom Antenna System - Gyroscope stabilized.
LST-5 Satcom Radio with D&M C152-1-1 Antenna
LST-5 UHF radio with D&M C152-1-1

Microwave

Satellite

When it was discovered that the cable companies were getting their programming by means of C-band (3.7 to 4.2 GHz downlink) receivers there was a move to make home brew systems.  The cost of a parabolic dish goes up as some power of the diameter.  The cost of a low noise amplifier goes up as some function of how low its noise figure is.  When the sum of the antenna cost and the low noise amplifier cost are plotted the curve has a sweet spot.  Over time the sweet spot has moved in the direction of smaller diameter antennas and better low noise amplifiers.

The best F/D (focal length of the antenna divided by it's diameter) for a parabolic antenna to have maximum gain is about 0.45, so most TVRO antennas were built with this rather flat shape.  The best G/T (gain divided by noise) is at an F/D of 0.3.  This is really the key specification since what the receiver wants is the best possible G/T.  I got one of the antennas from Stanford used for a study of the Sun (they had 16 in a N-S line and 16 in an E-W line all phase matched feeding a common receiver and all driven in hour angle to track the Sun.  These were spun Aluminum with an F/D of 0.3.  The feed was modified by an engineer working at Lockheed in Sunnyvale (government satellites) from a classical horn feed to what I call a nozzle feed that had matched E and H plane radiation.  This antenna out performed many commercial units with diameters up to 16 feet.  It used a WR-284 waveguide "button Hook" feed rather than use rods to support another type of feed.  It had only one polarization.

At first all the TVRO satellites used Vertical polarization.  Soon someone figured out that they could get twice as many satellites in the same angular space if the polarization alternated from horizontal to vertical.  The cross polarized signal is down 20 or more dB depending on the quality of the antenna systems.  Today this is still done on the Ku band TVRO 18 inch dishes.  It's too bad that they did not change over to circular polarization for the Ku band systems.  That would make VSAT type uplinks much easier to do since the "cross pol" adjustment would go away.

Radar Warning Receiver

I worked on some of the microwave parts used in these systems.  They used four cavity backed spiral antennas on each corner of an aircraft, each feeding a multicoupler with 3 or 4 outputs, each exclusively covering a different radar threat frequency and on each of these outputs was a Limiter-Detector that I designed and oversaw the production of. 

Spiral

Cavity backed spiral antennas (Wiki) were common on airborne radar warning receivers.
2863145 Spiral slot antenna, Turner Edwin M (Air Force), priority: Oct 19, 1955, pub: Dec 2, 1958, 343/767, 343/908, 343/895, 343/732 - 30 to 15,000 MHz
2947000 Beacon antenna using spiral, Kaiser Jr Julius A, Marston Arthur E, Jul 26, 1960, 343/895, 343/846
3717878 Spiral antenna, Mosko J (US Navy), priority: Jan 31, 1968, Pub: Feb 20, 1973, 343/895 - a lower frequency in smaller package

Direction Finding

There are a number of ways to get a bearing to a signal:
Doppler - uses a rotating antenna element and measures the Doppler shift
Pseudo Doppler - switches antenna elements to simulate a Doppler - BM Engineering out of business 1998-
patent 4,475,106 "High sensitivity portable radio direction finder"
Goniometer - rotating central commutator with multiple directional antennas like in the Wullenweber (Elephant Cage).
Amateur Radio Direction Finding Web Ring -
IDA stands for Information Dense Antenna - Roger Karlsson - 6407702  Method and system for obtaining direction of an electromagnetic wave
Loop antennas have a sharp null that can be used to get a bearing that's either to or from the station.  An Adcock (Wiki) arrangement of verticals also has this null property.
Fenwick patented a time delay beam steering system where the direction is frequency independent unlike phased arrays where the beam angle is a function of frequency.

Receiving Loops

ARN-83 LF aircraft DF set
ARN-89 LF aircraft DF set
AT-339 VHF low band hand held DF loop for PRC-6, photo
AT-784/PRC directional Loop for use with PRC-25
DU-1 Loop Ant and CRR-50001 Coupler Unit - Diagrams 1, 2, 3 & 4
MN-24C manual rotation aircraft loop - photo -
PRD-1 HF DF set
Radio Receiving Set AN/TRQ-23 and Antenna Group  OE-4 DF set with up to 150 RPM loop rotation
URM-6 Field Strength set 14 to 250 kHz

Other DF

Time Delay Beam Steering - see below for antenna system for Rx & Tx to a azimuth & elevation
Goniometer - This is a Watson Watt (Adcock) array of 4 vertical dipole antennas designed for use on a ship. Photo
Antenna
Goniometer
NUS-883   Serial 54-8
Federal Electric
Manufacturing, Co. Ltd.
Montreal, Canada
It is made to go on top of a mast with a 2" O.D.
It has 4 each vertical dipoles 50" end to end that are in a square pattern 14" on a side.
A Type-N connector and a male 10-pin military connector.
There is a lamp at the top that I think is a 110 Volt unit.
 I am looking for more information about this Goniometer Contact me

Antenna Theory

also see my Electronics web page.

Antenna Related

Supports

In many cases how well an antenna works depends more on it's support than on the antenna proper.  This is especially true for antennas operating above 30 MHz, i.e. where the ionosphere is not going to reflect the signal and to get good line of sight requires height.

AB-1135

This is a stake to be driven into the ground with the same antenna mounting block as is used on the PRC-25, PRC-77 and other radios.  The AB-591 spring base and AT-271 antenna can be mounted and then fed using a 50 Ohm coax transmission line.

Tripods - camera, survey, GPS antenna, Binocular

Pre-Amplifiers

When working at VHF and above frequencies the loss in the lead in coax becomes high.  There is a world of difference when a pre-amplifier is placed right at the antenna so the system noise figure is established at the preamp, rather than have the coax loss be added to the receive noise figure.  The Radio Shack 15-1170 works very well for this application.  Some of the other Radio Shack antenna amplifiers may cost more, but they are not continuous frequency coverage units like the 15-1170 MHz that work for a very wide frequency range

Antenna Tuners

This is a device that is connected between a transmitter and the feed line going to the antenna.  Typically there are coax connectors for in and out.  The military called them line flattners.  They transform whatever impedance that is seen looking into the feed line into 50 Ohms to provide the transmitter a good match.  The problem is that a high VSWR on the feed line results in a lot of signal loss.  These are rarity used for military applications.

Antenna Couplers

This is a tuner that is placed at the base of the antenna.  It has a coax feed to the remotely located transmitter and a high voltage insulator to connect directly to the base of the antenna.  An example would be the SGC - SG-231 Smartuner.  You will see these used for military HF radios that are man portable, on land, sea and airborne platforms.  There needs to be a remote control capability or built in intelligence to control how the coupler tunes the antenna.

Some of these antenna couplers use vacuum relays or motorized vacuum capacitors or roller inductors in their design.  All of these are very high Q devices and so the power loss in the coupler is minimized.  Other antenna couplers use fixed inductors and capacitors and switch them using relays.  This type of coupler has higher losses than the larger higher Q military type.

High speed couplers are needed when Automatic Link Establishment (ALE) is being used because the transmitter/receiver needs to tune to different frequencies quickly.  This means antenna couplers that use motors will be too slow.

Other antenna couplers are on my NRD 545 page

Antenna multicouplers

When you want to connect a number of receivers to a common antenna there is a very noticeable signal loss if you connect them all in parallel.  Instead what is needed is an antenna multicoupler.  They come as passive and active types.  There a number of government agencies that use these.  Some of the military Antenna multicouplers are on my NRD 545 web page.  I use the Stridsberg Engineering  MCA104 amplifier HF 4 output unit, it is hand sized rather than a rack sized military unit and has good specs and most importantly it works very well.  It's powered by a 12 Volt wall wart, but could be powered from a vehicle DC system.  It's driving my NRD-545, RCS-5A chirpsounder, McKay Dymec DR-33 receiver and Agilent 4395A spectrum analyzer.

A home brew 4-way GPS multicoupler based on Radio Shack satellite TV components works well and is a lot lower in cost than a "GPS" multicoupler.

Duplexer

When a single antenna is to be used with two transmitters or transceivers a duplexer is used.  It contains two cavity notch filters. The filter connected to radio #1 is tuned to notch out the transmissions from radio #2 and vice versa.  These are used at most frequencies from HF and up.  CU-2194 is a VHF Low Band unit that gets it power from the transmitted signals so does not need any batteries.Amateur radio repeaters use duplexers that use large cavity resonators.  The transmit and receive frequencies need to be separated enough so that the two filters do not overlap.

Time Delay Beam Steering

The "Beam and null switch step steerable antenna system" U.S. patent - 4,063,250 by Richard C. Fenwick is a great explanation of time delay beam steering.  The "phased array" antenna is a special case of the time delay steered antenna.  Note that in the time delay steered array the direction of the maximum lobe is frequency independent whereas in the phased array it is a direct function of frequency.  Fenwick also patented a "Three band monopole antenna" 4,145,693 that was sold with the beam steering network by "Omega-T".   This antenna has vertical polarization on the lower bands where the upper elements act as a capacitive top hat but on the two highest bands the upper elements act as half wave dipole antennas with horizontal polarization.

Counterpoise

Wiki says "used with radio transmitters or receivers when a normal earth ground cannot be used because of high soil resistance or other reasons".

This comes up when the antenna is not symmetrical.  For example a dipole in free space is symmetrical and so needs no ground or counterpoise.  A DX dipole, i.e. up 1/2 wavelength is close to a free space dipole and needs no counterpoise.  But if a dipole is being used for NVIS operation up 1/4 wavelength and the dirt (or sand) has poor electrical conductivity then a counterpoise (reflector) will greatly improve NVIS operation.

If  you insert an imaginary ground plane through the center of the center insulator of a dipole, throw away the left side and rotate the remaining 1/4 line counterclockwise you end up with a 1/4 vertical that needs to work against ground, i.e. an antenna that's unsymmetrical.  The e-filed lines should look the same as before for proper operation.  That requires a reflector at the ground plane.

If the dirt is of poor electrical conductivity, like sand, then the antenna will not work as you might expect.  A counterpoise is what's added to get the unsymmetrical radiator to work sort of like it's symmetrical counterpart.  There are a number of ways of doing that.  A solid metal sheet works very will, for example a metal car roof.  The metal does not need to be solid, so a screen where the length of the holes is say less than 1% of a wavelength would also work well.  For a 40 meter wavelength that would be 0.4 meters or about a foot. 

Another approach is to use a number of 1/4 wavelength radials.  For example AM broadcast stations do that.  To get the electrical length correct some tweaking is needed because of the properties of the dirt.

Another approach used by pedestrian mobile operators is to drag a wire.  It does not need to be a 1/4 wavelength because of the capacity of the wire to the dirt, so it's common for the length to be much shorter.  Dragging multiple wires or a plate may allow an even shorter wire to work well.

Concrete

Concrete typically has either rebar or wire mesh embedded to minimize cracking.  It's pretty easy to make an electrical connection to either of those, but can be a lot of work to be sure they are all bonded to each other.  Note:  You do NOT want any wire mesh or re-bar to touch dirt.  For example a ground rod under concrete that's connected to rebar is a very bad idea.  That can lead to the metal rusting away leading to failure of the structure.  There is a housing development built on the mud flats adjacent to the San Francisco Bay that had that problem.

Pattern

Classically the pattern of an antenna is presented in two ways.  The azimuth pattern is a circular plot of gain at some specified take off angle.  The elevation pattern is a half circle plot of gain.  Both plots are at some specified frequency.  For example see the TCI 651T plots.

Recently (Dec 2016) there's been some discussion about using drones (quad copters) to make HF antenna pattern measurements.  A number of prior art papers have been cited:

Beam calibration of radio telescopes with drones, Chang et al, 2015 (1505.05885v3.pdf) - a microwave horn fed from a noise source is used on a hexcopter to send to ground based 5 meter dish antenna.  The drone flew close to the Fraunhofer distance (Wiki, antenna-theory) which for a 5 meter dish operating at 1 GHz is 167 meters.

Measurement and Modelling of HF Antenna Gain and Phase Patterns and the Effect on Array Performance, Jenkins & Petrie, 1997 (00608612.pdf) - describes the Xeledop system.  Uses airplane with UHF differential GPS corrections to improve accuracy.

A comparison of modeled and measured HF antenna array patterns (Xeledop) , Jenkins, 1996 (P502025.pdf) - an airplane was used to fly a transmit antenna (selectable H or V polarization) around a ground based antenna and generate the pattern diagrams.  The modeling was done using NEC2.  The modeled result had higher errors ( p to 14%) at the lowest frequency (5.1 MHz).  Ground conditions such as freezing also have a big impact on the actual pattern.

In order to calculate the far field distance for an HF wire antenna, like a dipole the three conditions from antenna-theory should be applied:
1. R>(2D^2)/(WL)  - Fraunhofer distance
2. R>>D  - size of antenna ( for a dipole it's WL/2 for the antenna so 5*WL overall)
3. R>>WL this says at least 10* WL of 150 meters, or 1500 meters  (a little short of a mile) for 2 MHz. 

Patents

The US patent office is full of antenna related patents.  I have found that there are some great ideas that have never made it into text books, maybe because of the patent protection.  My Electronics page Antenna section has a number of patent links.  The US patent office has greatly improved in the last couple of years and now has ALL patents on line.  Prior to some year (in the 1970s) they are only available as images so are only accessible by patent number or class.

Google patents has used Optical Character Recognition (OCR) to scan all the US and many world and foreign patents so you can search Google patents for key words, that's something you can not do at the USPTO web site.  But the OCR is far from perfect so the search strategy needs to take that into account.  2016 update that's still the case.

Links

Shortened Antennas for Portables by Dennis Starks
Army Mars - antenna page
Antennas - active loops and whips by Charles Wenzel
Back to Brooke's Products for Sale, Military Information, Home

Page created 1 Nov 2001.