Novel Gamma Match Construction
Introduction
One of my early observations of Amateur Radio practice was gained during many visits to fellow hams to provide another pair of hands to assist with antenna lowering and raising. This ritual often involves maintenance of the existing antennae and reinstallation, and I made the observation that frequently, the construction practices and designs were the major cause of the need for maintenance.
A most consistently unreliable element of VHF Yagis was the common construction of Gamma Matches. They were very prone to corrosion and ingress of water leading to measured high VSWR and obviously degraded efficiency.
Gamma Match problems
Gamma Matches were appealing to hams looking for a means of feeding multi element parasitic arrays and were widely deployed.
The constructions fell into two broad categories:
 | a small adjustable capacitor usually housed in a nearly watertight plastic box with an adjustable tap point to the driven element and necessary supporting insulators; and |
| an adjustable coaxial tube capacitor with an adjustable tap point to the driven element and necessary supporting insulators. |
The first of these methods was difficult if not impossible to protect from ingress of water and dispensed with when the enclosure was seen to be more effective at trapping and retaining water than at excluding it. Condensing vapour traps are a risk of any kind of outdoor enclosure design.
The second method was usually ultimately selected as the improved design, but was still prone to problems to some extent. The specific implementation problems often included:
| electrolytic corrosion accelerated by contact between dissimilar metals which was a major problem in seaboard or acidic city environments; |
| metallic connections that were degraded by oxidation of the metal surfaces within the joint (especially with aluminium) and developed high resistance (loss); |
| insulating support structures that were deployed between relatively high voltage elements and fabricated from materials (plastic, wood and wrappings of electrical tape) that degraded quickly and either failed mechanically or became leaky (loss); |
| unstable physical construction that resulted in loss of tune to to progressive movement of parts. |
Design criteria
I wanted to make an assembly that was:
| mechanically robust; |
| avoided metal to metal connections that could develop high resistance; |
| minimised insulating structures; |
| was broadband. |
I was prepared to sacrifice the ability for inservice adjustment on the grounds that if it met the above criteria, it should not need inservice adjustment.
A different approach
To find a better way of doing things, you sometimes need to turn a design on its head. Well, what I did here was to do it back to front anyway!.
I designed a gamma arm that contained a coaxial tube capacitor, but differently, the coax cable was attached to the inner tube, and the outer tube of the arm was bent in an arc to its contact with the driven element and welded. The inner tube was copper, and the outer tube was of the same metal as the driven element (aluminium in my case).
Features of this construction are:
| the gamma arm was robust and self supporting requiring no support insulators; |
| there was no contact between dissimilar metals (wires, tubes, screws or rivets); |
| there was only one joint which needed to be sealed, it was at relatively low voltage and could be readily sealed with silicone sealant; |
| the gamma arm was the same diameter as the driven element resulting in a relatively broadband transformer. |
The match was relatively easy to construct, but I did need to prototype the match arm with an adjustable one which I discarded when the correct dimensions were found. The prototype arm used copper tube for the outer and was soldered to a worm drive hose clamp for an adjustable fixing to the driven element.
The above diagram is also available in Acrobat PDF format.
I made two identical 4 element 144Mhz Yagi arrays in 1970 using this design. Both antenna were matched to VSWR better than 1.1 using a Bird 43 directional wattmeter.
The test of time
The test antennae were intended as portable arrays for field use and could be stacked using a matching harness.
They have both been subjected to a fair amount of physical jostling and been out in the weather continuously for 28 years now.
Both antennas are perfectly serviceable, show no signs of corrosion or water ingress and still match at better than 1.1.
The following picture is of one of the antennas. It was constructed using hardware purchased from a TV antenna manufacturer. You will note that the steel assembly hardware (which was plated and coated) has not stood the ravages of the weather as the gamma arm. (The coax has been replaced recently).
Future
I have plans to construct an identical antenna using stainless steel.
'What about the skin resistance' you might ask.
Well I wonder about the skin resistance of natural aluminium which has acquired some surface corrosion over time, especially if in a salty environment. I think it will be interesting to see if a difference in a practical antenna can be observed.
Comparing stainless steel to aluminium, stainless steel is:
| easy to TIG weld, |
| stronger allowing lighter gauges; |
| a little heavier (when using lighter gauges); |
| electrolytically inert; and |
| higher electrical resistance. |
Advances in inverter based TIG welding equipment mean that construction of delicate assemblies is within reach of many home workshops. Alternatively, stainless is easy to gas weld, much easier than aluminium which soaks up heat, doesn't change colour much as it melts and has really low viscosity when molten.
The question is will it take me another 28 years to report my findings?
Remember, there is still room for experimentation in Amateur Radio.
Update - May 2007
After decades in the weather, the coax connection on the antennas needed replacement.
The picture above shows the revised feedline connection which has been applied to both antennas. The vertical aluminium plate was cut with a holesaw to fit around the boom, and is secured with two stainless steel screws. An N type connector is fixed to the aluminium plate. Conductive paste was used between the plate and element, and N type connector and plate. The Connector and gamma arm inner are sealed using silcone sealant.
The antennas are now 38 years old, and the gamma arms have never required repair or adjustment. The plated steel fasteners have been de-rusted and treated with fishoil to extend their life.
Last update: 29 May 2007 22:10
