THE CLEMENS MATCH
The Clemens match is easy to make and reliable, and is preferable to a single Gamma match as it is balanced.
The cable clips used are the black polythene types used to secure steel wire armoured cable to walls etc.
The length of the copper tube for matching at 50.110 MHz is 5.98683 metres * 0.125 = 0.748 metre or 29.455inches.
Put the Clemens match on the underside of the driven element to prevent large birds from damaging it.
I think it is preferable to connect the feeder braid to the centre point of the dipole also, as this helps to prevent unwanted currents flowing in the outer back to the tx.
Water proofing the joints etc. is very easy if plenty of polyeurothane varnish is painted on to them. This must be checked yearly to check for any signs of cracking or flaking.
I use LDF250 feeder from my rotator to the Clemens as this makes construction very easy. The outer plastic sheath is stripped from where the coax joins the dipole element. This gives continuous contact of the outer with the element all of the way along. Once liberally painted with clear polyeurothane varnish it is totally water proof, and corrosion will not take place.
Tuning is a matter of starting with the copper tube about 4 inches (10cm) away from the driven element, with loop of RG58 inner soldered to the copper tube at one end, and to the inner of the feeder at the other end. Attach another length of RG58 inner to the driven element at the other end of the copper tube. Make it about 12 inches (30cm) long. I secure it to the element by trapping it under a stainless steel jubilee clip. Using an antenna analyzer, (or low power tx and swr bridge) set to the design resonant frequency of the antenna. Push the RG58 inner into the copper tube until a dip in the swr is seen, then move the copper tube nearer to the driven element by a few mm. Move the RG58 in or out of the copper tube until a further dip an the swr is seen. Continue this process of moving the copper tube, and moving the RG58 inner in the tube until the lowest possible swr is obtained. Trim the lengths of the RG58 at each end if the loops in them become too large, until the final setting is arrived at. Finally, if you have left the driven element length slightly long as suggested, trim it by removing 1mm or so from each end and retuning the Clemens to the lowest possible swr. Do this very carefully, as you cannot easily make the driven element longer again if you cut it too short.
This process should be carried out with the antenna mounted so the reflector is 2 to 3 feet (55 to 90cm) from the ground and the rest of the antenna pointing at the sky. Next, before you finalize the Clemens, hoist the antenna in the air and find out how far the resonant frequency has moved. It usually drops by 50 to 100KHz. For this reason it is better to tune the antenna initially at a frequency of 50.210 MHz (for instance) so that when in the air the antenna is at the correct resonant frequency. This prevents the need to hoist the antenna up and down several times.
If you are satisfied that all is well, having connected the antenna to the feeder that will be used, and checking that the swr measured at the rig in the shack is no different to that obtained when tuning near the antenna. You can then substitute the pieces of RG58 inner used for tuning by a couple of new pieces cut to the correct lengths. make sure the loops at the ends are not excessive. Once every thing is in place, paint all the joints and exposed coax ends with a liberal amount of polyeurothane varnish. Do not paint the whole length of the copper tube as the resonant frequency of the matching will be moved slightly by the dielectric constant of the varnish.
Check, and check again that the swr is ok (it should be 1:1). Once satisfied you can install the antenna in it's final position.
The resonant frequency will alter slightly in wet weather and will tend to drop a few KHz.