Fox Hunting AKA Direction Finding or DFing Antennas

The ARCA held its first short range 2-meter fox hunt on the 23rd of July.  The great interest shown in conversations prior to the hunt, and also at the July club meeting, resulted in a poor attendance at the hunt.  For most people, it is simply a lack of DF equipment.  Here is a listing of some commercial DF kits and some instructions on how to build an inexpensive DF so you can join the fun.

Ramsey Electronics of Victor, N.Y. (1-800-446-2295) offers two DF kits. The Ramsey DF-1C is a hand-held lobe switching kit that sells for $74.95 plus shipping. Henry, KN4AV, has one and you can talk to him about the unit.  The Ramsey DDF-l is a Doppler automatic direction indicating mobile kit that sells for $169.95 plus shipping.  There is a military surplus DF antenna that you might find, the AS-2365/GR, that works on 2 meters.  The Direction Finding Antenna chapter in the ARRL Antenna Book has instructions for building several units if you want to roll your own.

If you want the simplest, cheapest, and easiest to build DF, here is a set of directions. Like most DFs, this home-made one is bi-directional, with two nulls 180 degrees apart. There are no electronics:  just two coax-fed dipoles, approximately a half-wavelength apart, connected in parallel. One is mounted upside down to provide a 180 degree phase shift.  When the incoming signal hits both antennas at the same time, they will cancel each other out and provide a null which points the way towards (or away from) the incoming signal, which is at a right angle to a line through the two antennas.  If you want a name for it, it is a Michelson Interferometer Radio Direction Finder.

Materials needed to build the unit are PVC pipe (1.5″ or 1″), a Tee fitting, two dipole antennas
(telescopic whips, solid aluminum ground rod wire, silicon bronze welding rod, or number 10 or 12 solid  copper wire), RG-59 coax, RG-58 coax, and a coax connector to your HT.  The drawings show the wiring and construction of the antenna.  A variable is how you build and mount the antenna elements at the ends of the horizontal PVC pipe.  Nothing is critical about the mounting or the lengths of the elements, as long as all antenna elements are the same length.  Also, the
two RG-59 coax segments from the antenna elements to the splice must be the same length.
The RG-58 only needs to be long enough to reach your HT when you hold up, extend and rotate the antenna.

Chuck Teeters – W4MEW
Published in the August 2005 edition of The Splatter

Please note that Ramsey Electronics is no longer in business and does not sell the kits mentioned in this article.  However, these kits may be found for sale online.

Radio Direction Finding

There’s been talk lately about Club-sponsored “Fox Hunts,” or transmitter-hunts (T-hunts).
I have no idea how many Club members have had experience with Radio-Direction-Finding (RDF), but it is a valuable skill. Besides the fun of an actual contest, T-hunting skills are helpful for finding a signal that is, say, jamming a repeater, or perhaps power line or other interference, or even Cable TV “leakage.”  Some T-hunters even use their skills to work with the Civil Air Patrol or the US Coast Guard Auxiliary to help find lost or downed planes.  A “Fox Hunt” helps to build RDF skills.

The required equipment is somewhat specialized, but it’s easily obtained or assembled, using the literature which is found on the subject.  For instance, the ARRL website has several good articles that describe different kits, which are generally still obtainable from the authors.  The articles also explain the techniques needed to master the art of finding a transmitter which may be on for only limited periods, and whose signal is hidden among multi-path reflections.  A T-hunt can be done in any kind of area and terrain.  For instance, it may be limited to a small park, where the “fox” has only a few milliwatts of power, or it could be “unbounded” – like some contests that go on in California – where the hunt could encompass as much as 400 miles.

Whether T-hunting by car or on foot, at the end, it will pretty much always require some hunting on foot.  So the equipment you use must be adaptable for utilizing in a car or on foot-which is not always practical or even feasible.  And, the equipment used in a car must be “operator-friendly” – so as to avoid creating a collision.  In the final analysis, the equipment used in a car is different than that used on foot – which is why there are so many designs of the same basic equipment.  If a vehicle is needed for the T-hunt, then it is usually necessary to have some portable equipment that is lightweight and easily used while running through undergrowth in the dark, for the final leg of the hunt.

RDF equipment falls into 2 categories.  The first is an attenuator, so that your S-meter doesn’t peg when you get close to the transmitter – and to help give you an indication of range.  The second, and more complex is the actual direction-finding equipment, which is to point you the right way. Essentially, it can be done by using a directional antenna (or a means of simulating one), taking a reading, then moving some distance and taking another reading.  By “triangulating” in on the signal, you determine which direction to move.  S-meters and/or LED displays, or even audio tones, are generally used to indicate direction, while S-meters or audio tones are used with an attenuator to indicate signal strength.  Much of the mobile RDF equipment utilizes multiple antennas, which are rapidly switched in & out to simulate a rapidly rotating antenna.  This allows utilization of the Doppler effect to determine directionality.  Other means are “balance” detectors, which utilize dual antennas for right-left determination, or 4 antennas for right-left and front-back.  When on foot, it’s generally enough to use 2 antennas for right-left, and use your body rotation to determine front-back.  The “HandiFinder,” a handheld RDF device, is based on that principle – it produces an audio tone when one antenna receives the signal stronger than the other; loudness (in either earphone) indicates the amount of un-balance; a “null” produces no audio.  Practice with this device will allow the operator to determine direction on foot very rapidly.  An attenuator is helpful to determine distance from the transmitter.

So, you can see where there is lots of potential for developing skills, using various RDF devices, and for determining which ones are most useful at which times.  Building kits, or devices of your own design, is also fun. Developing improvements on existing devices is always useful and fun.
Usually contest rules allow working in teams or as individuals, but small 2 or 3 man teams are most
fun and are usually necessary to handle all the equipment.  Working in teams is also a good way for us to get to know each other better, and to build teamwork.  Things can become very competitive, so, your team should always remember the adage, “Never trust anything said by a T-hunter or a T-hider.”  Let’s get out and have some fun!

Chuck Teeters – W4MEW
Published in the August 2005 edition of The Splatter

The Trouble With J Antennas

The J antenna is a simple, omni-directional, vertically polarized, coax-fed antenna, with some gain, doesn’t have radials to get in the way, and is easy to build…BUT.  The trouble with J poles is the
hams building them (and most of the instructions, e.g., see QST’s copper tubing J poles) don’t know how they work. As a result, they are not omni-directional, and they do not have any gain over a ground plane antenna.  They certainly work, but not nearly as good as they should.

The J uses a vertical half-wave antenna, which has all the above-mentioned advantages, except it is not easy to feed with coax.  The J uses end feed to the dipole, which is a high-impedance feed, about 2400 Ohms, and you cannot feed coax to it. Therefore a J pole uses a quarter wavelength of open line as a matching transformer between the coax and the bottom of the half-wave vertical. The quarter wave matching section provides and impedance transformation of 50 Ohms to 2400 Ohms…IF…it is built correctly.  If it is not built correctly, it is a poor matching section and becomes a part of the antenna.

This is because open transmission line matching sections will act like an antenna if the line is spaced over 0.02 wave lengths (see ARRL Handbook, 1941, p.122 and in every issue since).  A separation of 0.02 wavelength on 2-meters is 1 – 1.5 inch. That means the separation between the two wires or tubes at the bottom of your J should be under 1.5 inch; If it is more, the bottom matching section will radiate RF just like the antenna.  Three bad things happen when the matching section radiates.  First, the two sections, the half-wave and the quarter-wave, radiate RF energy out of phase with each other and a lot of the RF cancels out, making your signal weaker.  Second, a matching section does not radiate an omni-directional signal, since the two wires or tubes are displaced from each other. Therefore your J is not omni-directional.  Third, a radiating matching section does not provide a good match between the coax and the bottom of the half-wave antenna.

Results are poor in some directions, and the coax must be moved up from the bottom of the
matching section to get a low SWR.  So, if you have these problems, take a look at the spacing on the matching section.  I’ll bet it’s too wide.

Merry Christmas!
Chuck Teeters – W4MEW
Published in the December 2005 edition of The Splatter

SWR Meters and Radiated Power

As W1SUJ recently demonstrated, a low SWR is not an indication of antenna performance.  Richard got a 1.2:1 SWR on 2 meters with his new antenna, but could just about make the club repeater.  SWR is supposed to tell you if the antenna is matched to the feed line, but the SWR meter is at the transmitter end of the line, so it measures the forward power coming out of the radio, but the reflected power is that that goes from the radio up the transmission line to the antenna, and is reflected back towards the transmitter, and goes down the transmission line again, so if there is loss in the transmission line (and all lines have loss) the reflected power at the SWR meter is way less than the real value.

For example, lets say we have 100′ of RG-8X.  At 144 MHz this has 4 dB loss.  The balun, antenna switches, and connectors may add another 2 dB.  So we have 6 dB loss and therefore only 25% of our power gets to the antenna.  Lets say because the antenna is defective or out of resonance, the SWR, at the antenna, is 10:1.  So the antenna only accepts 18% of the 25%, or 4% of the transmitter power.  The rest of the power is reflected back down the line.  So the line loses 75% of the reflected power.  This means that the SWR meter sees a reflected power of 25% of 82% of 25%, or 4.5% of the original power.  Therefore, the SWR meter sees 100% forward power and 4.5% reflected power and reads 1.1:1.  With this SWR you would expect everything to be great, but using a 25 watt transmitter you would be getting only 1 watt into the antenna.

At the same time, the antenna may receive great, as the SWR (and loss) when receiving is determined by the match between the transmission line and the receiver, and will be entirely different from the transmitting SWR.  So, as Larson E. Rapp (WIOU), would say, “in hock vertis” and build or buy a field strength meter and find out what is really going out…

Chuck Teeters

Bigger is Better

Some of the antenna stuff you hear on the air these days makes you think that 2 plus 2 now equals 5. A few basic facts about radio waves and antennas are therefore needed. If you look around at the radio waves passing by, you will see some strong ones, some weak ones, and some in between. The way you measure a radio wave is in volts, just like a battery, except RF volts. Since the radio wave doesn’t have terminals on the end, we pick some spots on the wave to measure the voltage. The normal measurement is between two points on the wave on meter, about 39 inches, apart. The voltage is not much, so we usually measure millivolts or event microvolts. We have to measure the wave in the plane of polarization, if it’s vertical we measure the vertical meter distance, and if it’s horizontal we measure horizontal. The result is that we get a measurement of so many microvolts or millivolts per meter. For example, if you measured a broadcast signal from a 1000 watt station at one mile, you should get about 150 mV/meter. A TV station at 30 miles should measure about 3 mv/meter. Most ham band signals will measure between 1 and 100 uv/meter, if you are a few miles away.

Now how much signal does your antenna get from that radio wave? It depends upon how long it is. If the signal is 50 uv/meter and your antenna is 10 meters long, in the plane of polarization of the wave, you will get 50 x 10 = 500 microvolts into your receiver. If your antenna is 50 meters long, you will get 2500 uv. So, the longer the antenna, or the stronger the radio wave, the louder the signal. So also, small antennas = small signals and big antennas = big signals, no way around it, so hang up some wire.

Chuck Teeters