The RTV80

Overview

Some background

One evening on 3.787.00, several hams were involved in a round house when Dave, W4UWH, made a comment about installing an adjustable dipole on his new tower. He was at his winter place in the north Georgia Mountains where he was in the process of building a 150 foot tower. One anchor point is south facing and runs down slope for several hundred feet. The guys are phillystran so are ideal for supporting an antenna. The discussion turned to tuning and how the antenna could be adjusted from a remote location. Dave had considered using a stepper motor to feed out the wire and some sort of weight system to take up the slack. I thought any electrical device at the antenna was asking for trouble. That started us thinking about how else tuning may be done. Sam, (KC4TAQ) and I (W4QJP) have built several inverted Vs for local hams and our procedure is to cut the wire longer than needed and start folding back the ends in equal lengths and wrapping them around the other wire until the analyzer shows we have a match at the chosen center frequency. Some antenna books recommend shorting the ends of the wire back to itself which we’ve never done and suffered no ill effects that we could observe. We theorized that if the wires were kept in close proximity to each other then it would still appear as if that amount of wire was folded back or cut off the ends of the antenna. I booted up the computer, brought up my favorite drawing program and proceeded to illustrate our version of the antenna. I emailed it out to Sam, Dave and several of the others that were in attendance at the time of the discussion to get their feedback. Thus began the search for a Remotely Tunable inverted V for 80 meters (RTV80). Sam spent several hours, as did I, searching the internet looking for information. It’s been said that nothing is original and all things have been done before. We found information on motorized spool systems and plenty of information on vertical antennas, such as the “screwdrivers” so popular for mobile applications, but not one word on our design. Over the following week I built a crude hand winch and picked up some 2” clothesline pulleys and a few other items. On Saturday, Sam, Gary (AJ4TI) and I installed version 1 on one of Sam’s towers using THHN #12 stranded copper wire. We discovered several things with version 1, among them, the lack of change in frequency as the antenna was shortened. The second thing we found was the tendency for the pulleys to flip over and twist the wires and Dacron rope together binding the take-up system. At the time we thought the insulated wire was the problem with frequency change and decided to change to bare wire. We change out the wire and added metal pulleys to short the ends together. We hooked up the analyzer and found the SWR varied from flat to a high of 1.3 to 1 in the middle of the band. We solved the twisting problem by taping garden stakes to each of the pulleys. We now had a working antenna that covered the entire 75/80 meter band. We know antennas don’t like company so we set up the PACRAT and moved the antenna to the front yard to get it in the clear. We installed two 21 foot lengths of 2” pipes on the fence line at opposite sides of the yard to handle the weight system. The antenna liked this location even better than the last. The next step was to add a motor drive to take up or pay out the antenna rope. We requisitioned a DC window motor Sam had laying around and built a new winch around it. It turned out to be much slower that we needed but the Dacron rope level winds perfectly onto the spool, in part because of the distance from the top block down to the winch. We didn’t like using the stiff wire as it will work harden and likely break strands. We really wanted to use bare flexible wire but had concerns about the small strands abrading as it moved around the sheaves. While talking to Dave he assured us that insulated wire would work but it needed to be spaced close together. We made a trip to Skycraft in Orlando and bought several items including some very flexible #18 insulated wire, a length of Delrin rod, some Plexiglas and 3 DC gear motors. We turned several 1” sheaves and cut the major parts from the Plexiglas. We assembled and installed the new version with all the new parts and were pleased to find we now had an antenna that covered the 80 meter band at 1.1 to 1. Sam and I would like to see this antenna in use around the country and encourage anyone that may want to give it a try. You’ll find instructions elsewhere on this website with suggestions of ways to keep the cost down and we also offer the main parts for a nominal fee. George & Sam

W4QJP & KC4TAQ

NOT to scale

Designed, built and tested by W4QJP & KC4TAQ.