In the fall of 2007 I purchased a Mosley AM-56 50-MHz beam. I was in no hurry to get this up as there were zero sunspots, and I knew we would not likely see any sporadic-E activity until June/July of 2008. In April 2008 I assembled the antenna according to Mosley's instructions, picking the setting for 50.1 MHz (the two shorter element lengths were specified as resonating at 50.7 MHz and 52 MHz.) When I tested it, the lowest SWR was at 51.1 MHz, a full 1 MHz higher in frequency than documented. Because the elements came pre-drilled, I did no take the time to measure them the first day. I asked if anyone had similar problems, or if the fact I was measuring the SWR close to the ground was causing the resonance point to shift. I was assured that since I had tested it on a 14-foot pole, this was high enough and that raising it to 55 feet would not make a noticeable difference. No one had any obvious suggestions as to why it was not resonant at the proper frequency. It was getting dark so I decided to sleep on the problem.
As it turns out, the AM-56 is sold in both a Military and an Amateur version. When I measured the elements the next morning, I found they were about 2.5 to 3 inches shorter than the documentation. (This using the pre-drilled holes in the 50.1 MHz position.) My guess is that Mosley mistakenly sent me a version pre-drilled for military use (do they use frequencies in the 52-55 MHz range?) and sent the Ham one to the Forces. If so, I can envision last fall a GI was scratching his head wondering why the yagi he received didn't load right. Either way, setting the elements to the proper length and drilling a fourth set of holes to secure them from telescoping brought the resonance point down to 50.1 MHz where I wanted it.
Three "factory" presets and screw in the new fourth location
It was suggested to me by Dave W1MCE (a lifelong Electrical Engineer) and Paul W8AEF (a member of the 2008 TX5C Clipperton Island team) that you can test Yagis with a 50-ohm characteristic impedance by getting the reflector at least a foot or two off the ground and pointing it straight up. As explained by W1MCE, "The reason the straight up method is effective is that the antenna has a F/B ratio of >20 dB, plus or minus, that provides effective isolation of ground effects. Ground effect is isolated by the F/B ratio and has less than 1% impact [-20 dB] on tuning."
VE1DX patented antenna test range (built based on the "straight up" suggestion of W8AEF and W1MCE)
Although not perfectly 1.0 to 1 (I suspect some interaction from the swing set), the lowest SWR was now at 50.1 MHz. It pays to measure the antenna lengths to double check! I got a SWR of 1.5 to 1 at 50.1 MHz.
Lowest SWR at the expected frequency
Raising the antenna to 14-feet resulted in a SWR of 1.2 to 1, and I believe this improvement to be removing it from the proximity of the swing set as opposed to getting it higher and horizontal. Thanks to my 18-year old son Kevin for acting as the temporary tower and rotator. Regrettably he has to go to university so another means of antenna support had to be found.
1.2 to 1 SWR at 14-feet
The next day I installed the antenna about 9-10 feet above my Mosley TW-33-XL WARC band beam. This further lowered the the SWR to 1.1 to 1.
SWR to 1.1 to 1 (as measured at the rig with the beam on the tower)
The AM-56 has a unique feed system. The two phasing rods seen below are only electrically connected to the radiator (a/k/a the driven element.) They are physically attached to the underside of the first director, but are insulated from it. The only reason for connecting them is purely for strength and support. The following is copied from the Mosley instruction assembly manual and explains how the feed system works:
Since the feed system of this antenna is a somewhat modified version of usual matching methods, an explanation of its operation will clarify its working principle. The impedance measured at any two points on a half wave element will be of a value depending upon the distance from the center - with very low impedance near the center and rising rather rapidly as the connector is moved toward the end of the antenna. A quarter-wave stub of about 400 ohm characteristic impedance may be connected at a point on the antenna that matches the 400 ohm stub. The stub, if left open at the opposite end, will have very high impedance at this open end. Since the end connected to the antenna is practically shorted with respect to the stub, the stub will act as a transformer with the impedance very low near the antenna and rising to a high value toward the open end I transmission line of any practical characteristic impedance may be tapped on to the stub at a point matching the line. Such points are provided for lines of; (1) 52 ohms, (2) 72 ohms, (3) 104 ohms and (4) 300 ohms.
This is a balanced stub so some means must be provided to remove the outer conductor of coax from ground. NOTE: This may be accomplished by connecting the outer conductor of coax lines down the point to be isolated. This grounded outer conductor, along with the boom, acts as a balun to accomplish the balance of two ends of the coax line. The ground is omitted for balanced lines.
I had to read this a few times and look carefully at what Mosley are doing, but it makes sense when you think about it. Essentially you can attach your feed line anywhere along those phasing rods (quarter wave stubs) and, depending on where you do it, you will get various impedance values. It is a clever way to allow this beam to be fed with almost any value of feed line (52-ohm, 75-ohm, 300-ohm, etc.)
Feed system tapped at the 52-ohm point and showing the coax braid shorted to the boom.
(Braid is shorted at 42-1/2 inches back from the feed point)
Feed system waterproofed prior to installation on the tower
(Note the stubs are insolated from the director)
Initial testing with a 50-MHz beacon about 25 km (15 miles) away showed good front to back and side rejection. There is no activity on the 6M band right now, but when sporadic-E season starts in about 6 weeks, I'll put it through its paces. Mosley claims generous specifications for this antenna (10.8 dBd forward gain and 20 db F/B.) This is significantly better than other 5-element 50-MHz yagis on a 12-foot boom. Time will tell if this is overly optimistic, but I have had great experience with Mosley products in the past. Prior to purchasing this antenna I had several discussions with Gary Wurdack Sr of Mosley, and he stands behind the published figures. These values are derived at Mosley's testing ranges and are not taken from look-up tables or theoretical calculations. Because Mosley is a large supplier of Military and commercial antennas as well as Amateur equipment, they have to test their antennas to meet the required specifications. I'm confident this antenna will be a performer.
AM-56 50-MHz beam stacked above the TW-33-XL WARC beam
AM-56 - closer view