VHF Distant Communication or DX?

I have written articles in the past about Ham FM activity. I actually enjoy FM on 10 and 6 Meters, and I own three different repeaters within our VHF and UHF spectrum. This article though will be about DX. This means no wideband 16 kHz bandwidth Frequency Modulation! It's about 3 kHz bandwidth Single Sideband, and even narrower iCW! The object is to attain a good "Signal to Noise" (atmospheric noise) ratio.

VHF DXing can be very challenging, and immensely rewarding! It never fails to thrill and delight me when I work someone several states away, or a different country via a frequency that used to be channel 1 television! On the higher frequencies to work someone in another state via a 144 or 222 MHz, signal is tantamount to an experiment in physics! The wonder of working someone in Arizona or at the far end of California, yet 400 miles away can be done every day though, and I savor the science involved!

Therefore, what does it take to accomplish this? The major hardware is a good antenna, and that's not very hard to provide. Even a four element 2 Meter wavelength band Cubical Quad on a 66 inch long boom and fabricated of wood and wire will provide 10.1 dBd of gain. A 50-foot boom Yagi on 6 Meters is an antenna better than 2.5 wavelengths long. If you tried to build such a Yagi for 20 Meters, the boom would be just short of 176 feet! The elements on a 6 Meter Yagi are only about 9 feet long; the antenna is very small from a perspective of mass or wind load. On 6 Meters, it is quite common for serious VHF Ham's to stack and phase 2 or even 4 such Yagis. One though is quite good. Even two 4 element Yagis on 12 foot boom's can be a very effective tool, or one 6 element Yagi on a 24 foot long boom!

On the higher bands of 2 Meters and 1.35 Meters, antennas are very easy. A Yagi with elements only about 2 feet long, and a boom just slightly over 32 feet in length can provide 16.2 dBd gain! The same antenna if scaled to 20 Meters would require a boom length of about 492 feet! It is very feasible to assemble a phased array of several of these VHF antennas. Have you ever seen someone phase four 20 Meter Yagis with 400 foot long booms? Antenna height is also a very important consideration. Thirty feet above ground and other obstructions is the break-point for this installation engineering. Below thirty feet, VHF antennas will appear to lose gain. Above this point, signal gain is effectively improved. Be careful though because as this height increases, losses in feedline can offset gain!

Thus, feedline loss considerations are important for such antenna arrays, and it is good to deliberate and be conscious of this. Coaxial cable is often referenced with statements by some Hams who say 'you MUST HAVE high quality Heliax or hard-line coaxial cable to work VHF. If you look at these installations closely, you will find that they all have about 1 decibel of feedline loss, a few tenths of a dB more, a few tenths less.

You will also often hear of Hams using coax from "The Feedline Man" or some such manufacturer. Now maybe these vendors have good coax, and maybe they don't? I am always a bit skeptical when I hear about cable that is designed for superlative low loss, and it's super flexible too! That kind of reminds me of guys that get a Volkswagen, and then try to turn it into a racecar! I use Belden cable. This is a company that you may have heard of, since they have been around for many, many decades. I use their Military specification (Mil Spec C17D) cables such as part number 8267. Many people call this RG8! It is in fact RG-213, a variant of the older World War 2 Mil-spec (JAN C17) RG-8A/U that I used as a kid. I usually wind up using about 50 feet or so, and consequently have about 8/10ths of a dB of loss over that length. Belden specs say 1.6 dB of loss at 100 feet on 50 MHz. I don't object to the idea of better coax, I just weigh all of the factors. I do typically use "Constant Impedance" coaxial connectors such as Amphenol type "N"! I also replace coaxial cable every 3 to 5 years.

Another facet of VHF DX is the physical nature of the ways in which these frequencies behave. Max Planck taught us that each time we double frequency we double energy level. A simple demonstration of this is to take a small power output transmitter, just a few Watts, on HF frequencies and touch the tip of its antenna with your finger. You will either feel nothing, or a small tingling. Do this with the same power level on UHF frequencies and you will get a painful RF burn! These energy levels though are our friend! They help us to explore and utilize the characteristics of the upper atmosphere, such as using Tropospheric Ducting, or bouncing signals off the roiled and disturbed air that is excited in the wake of jet aircraft, or even bouncing signals off the Moon! That's what I mean about experiments in Physics! It's just fun to play with this stuff!

The atmospheric noise level is considerably lower over most of the VHF spectrum. This factor along with the energy levels that can be produced by even modest power stations allow signals to be bounced of the trailing hot gasses of jet aircraft as mentioned in the earlier example. One prominent California 222 MHz Ham has carefully documented this. Other propagation effects allow for working distant stations merely by tropospheric ducting pathways, or by lower ionospheric layers than the F layer. The E-layer, during the months of spring will support 6 Meter DX over thousands of miles. Even the atmospheric D-layer will also convey these 6 Meter signals during the strongest solar flares, or Proton events. We all know about F-layer DX, when the Sunspot cycle peaks every 11 years. It's those lag years in between though that bother me -- that's another reason why I like VHF and above! I don't need the F-layer, when the E-layer will work just fine all year and every year!

The F-layer, actually F1, and F2 layers reside at a height of about 150, and 200 miles respectively, and when they are ionized to the extent that they will provide a high order of signal refraction, they work very well. The E-layer resides about 85 miles up, and is often ionized by many more regular occurrences. This allows typical E-layer signal skip to support communication to about 1200 miles in a single hop. Multiple hop skip also happens very often supporting communication out to 2500, or even 3600 miles. I have in the past worked stations in South America via a probable combination of E-skip, and possibly some ducting.

Aside from these longer oriented signal aids, Troposheric ducting can support signal propagation out to several hundred miles. A good antenna, and a decently engineered station can work "Tropo" very easily. It does not require a huge investment, only a bit of education and desire!

The next method of VHF signal propagation is I think probably the most fun, that's Meteor Scatter! Meteor showers occur as regularly as sidereal time allows. They are notable at calendar dates, and can be counted on to provide good results. Even a quarter-wave Ground Plane antenna on 6 Meters works for this! My favorites on 6 Meters are:

Scorpiids June 2nd-17th

Pons Winnecke June 27 - 30th

Cygnids July 14th

Capricornids July 18 - 30th

Perseids July 25 - August 4th

Ursids December 22nd

The next and last requirement is the radio hardware. Well-engineered transverters are best, although on 6 Meters there are very many high quality transceivers. With one of these transceivers, and transverters for each higher frequency band to be engaged and explored allows excellent operative potential. These transverters are designed with superior quality front-end RF amplifiers for the receiver stage, and the transmitter power is already at a respectable 25 Watts. Linear power amplifiers are common with serious VHFers, and the best of them can produce legal limit power levels with only a few Watts of excitation drive. These utilize high mu ceramic tetrode vacuum tubes but, quite a few Hams use only Solid State 'bricks' of about 100 Watts output. Some Hams just use the transverter's basic output and with a decent antenna and do quite well!

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