Emergency Field VHF/UHF Transmitting Antenna

   As an ARES member, I often ask myself this question: “What would I do if I was deployed to an area but forgot to bring my antenna with me?”

After some research I found a very simple solution. It only requires a piece of coax cable and a few simple items to construct an effective transmitting antenna. 

(fig. 1)

Theory

As amateur radio operators, we know that one of the simplest antennas to construct is the dipole.  Invented by Heinrich Hertz around 1886, the dipole antenna has been around since the dawn of radio. The half wave center-fed dipole antenna has a feed point impedance around 73 ohms.  Z in = 73 ohms + j42.5 ohms.  We are seeing some reactance here. To reduce this reactance, the ideal length of the center-fed half wave dipole should be around 0.48λ which makes Zin= 70 ohm. Most modern transceivers have an output impedance of 50 ohms, so with the load being 70 ohms our rig will be happy with the SWR at around 1.25:1.

Using this theory mentioned above, we can easily calculate the length of the dipole we want to construct. The formula to find any half wavelength in linear feet is to divide 468 by the center frequency in MHz for the desired working frequency range. Most VHF FM repeaters are using 145 MHz to 146.9MHz, so let’s set 145 MHz as our center frequency.  Therefor, 468/145 = 3.23 feet or approx. 39 inches for our half wavelength in this case. The reason I set 145 MHz as center frequency is that in some situation you might need to use the weak signal portion (SSB/CW operation) of the 2 meter band which is around 144.200 MHz.

Now that we know the linear length of our half wave dipole, we can start making the antenna.You will need a piece of 50 ohm coaxial cable (RG-58, RG-8U, RG-8X, LMR-240, LMR-400, etc.). A piece of CATV 75 ohm cable such as RG-6 used by most home cable service companies will work in a pinch, too! Remember,if this is for an emergency situation,we use whatever material we can get our hands on!You will also need a short piece of ½”-1 ½ ” diameter of non-conductive material(PVC, wooden dowel, etc.) at least 12 inches long to hold your dipole on a mast.

Making the Antenna

Cut off the connector at one end of the coaxial cable if there is one.  Measure 19 inches from the tip of the cable. Circle-cut the outer PE jacket of the coax cable, being careful here not to cut through or nick the shielding of the coaxial cable.  Once cut, you can slide off the jacket and discard.  Carefully separate the shield from the insulated center wire down to where you did the circular cut.  You can accomplish this several way.  One way is to un-braid the outer shield down to the cut PE.  Another way is to un-braid a small section near the base of the circular cut, then pull the center insulated conductor through the opening.  Once it is completely separated the center from the shield.  It should look like this. Now your half wave dipole antenna is almost ready!

Next step is to fabric some sort of mounting system to support using the piece of non-conductive material as boom.  This will allow you to elevate and rotate your antenna.  You can use tape or other non-conductive fasteners secure your dipole elements to the boom.  Coil the coaxial cable just below the feed point 4-5 turns, about 6 “ in diameter.  By doing this you’ll create an RF choke to cut off the common mode currents that might develop on the cable during transmissions (see fig. 2).

 (fig. 2)

Common mode current in feedline(source: Wikipedia)

Possible Gain!

A ½ wave dipole has an Omni directional radiating pattern off the broad sides.  It has a theoretical gain of 2.15 db over ¼ wave antenna.  We can achieve get some extra gain and directivity out of this antenna by using a conductive mast as a reflective element, just like a 2 element Yagi-Uda antenna! I ran some simulation in MMANA antenna modeling software and see about 1.98 db. Front to back ratio is around 5.26 db, beam width is about 100 degrees.

In order to take advantage of the mast reflector, the placement of your dipole should be 8 inches in front of the conductive mast with the top-tip of your dipole being 4 inches lower than the top of the mast.

(fig. 3)

2m dipole free space pattern using metal mast as reflector

Dual band operation

How do you make this antenna work on 440 mhz? another popular amateur frequency, most importantly our (Nassau County ARES) main repeater is on this frequency. 440 MHz falls in the third harmonic of 146.7 MHz. So in theory the 145 MHz dipole is a 1.5 λ radiator on 440MHz.

(fig. 4)

The impedance of this setup on 440 MHz isn’t too crazy, at around 100ohms with some reactance. SWR will be around 2:1 to 3:1 which is still usable. But there is a very serious problem presenting with this setup: the radiating angel is too high! If we want to reach a distant location we need to focus the antenna pattern lower on the horizon.  It is more generally a rule that VHF& UHF propagation is strictly within the line of sight than HF signals. So a 45 degrees take off angle from this setup is unacceptable!

Free space pattern  for a 2meter dipole operating on 440MHz.

To fix this, you will only need another piece of conductive material.  You can use a coat hanger or any conductive wire. Straighten the wire and cut a piece measured 12.5 inches long. Mark the center of this wire and put it 1 inch in front of with the 145 MHz dipole.  Tape it down or use whatever method to secure the wire. This wire should be parallel to the 145mhz dipole. The added 12.5 inches element is called “open sleeve” radiator. Now the radiating pattern looks like this.

(fig. 5)

Dual band dipole 440 MHz free space pattern using metal mast as reflector

Summary

There are three advantages of this antenna.

1. No soldering required.
2. Dual band operation.
3. Achieve extra gain or directivity by simply changing the mast material.
4. Simple, inexpensive construction using readily available parts.

In an actual emergency situation this antenna can be quickly prepared and erected. Bandwidth is also adequate for transmitting on the 2m and 70cm amateur bands.