Thursday, October 20, 2011

A Remote tuned Dipole that covers ALL of 75/80 Meters without a tuner!

The RTV80
A Remote tuned Dipole that covers ALL of 75/80 Meters without a tuner!
Now one dipole for the entire spectrum from 3.5Mhz - 4.0Mhz!

Now you can have the entire 75/80 meter band at your disposal using one dipole
 without the use of a tuner!

How it works and is assembled:
HOW IT WORKS: version 1- refer to drawing above.
The wench at the bottom of the tower is used to take up or let out the Dacron rope that is routed up the tower and connected to a pair of Dacron ropes that pass through two pulleys near the apex of the antenna. Each rope is connected to the ends of the antenna wires which in turn are routed between a pair of sheaves to keep the wires close together and maintain electrical contact.
The wires continue down to the lower sheave or pulley, around and back through the upper pair and up to the feed point insulator where they are connected directly to the center and shield of the coax, the same as you would with a regular inverted V or dipole. The lower end of the take-up system is attached to Dacron ropes that pass through elevated pulleys and are then attached to counter weights.
The weights maintain tension on the wires as the wench lets it out or takes it up.

After the antenna was installed, an MFJ analyzer was used to find the lowest reflected power at regular intervals on the 80 meter band and the Dacron rope was marked with frequency tags at the wench. We can take up or let out the wench until we’re at the frequency desired.

This would be the budget version of the adjustable inverted V. You would do the frequency changes by cranking the wench handle at the base of the tower – maybe not so good on a rainy day.
If you desire to operate the antenna from within the shack, there are several methods that may be employed.
The redneck version would use pulleys to route the Dacron rope into the shack where you can directly control the wench and see the frequency tags. 
A counter could also be driven from the wench although a tracking system is unnecessary if you tune to the lowest SWR at the frequency desired. 
We added a 12 volt DC window motor to drive the wench and control it with a DPDT switch which takes its power from the same source that powers the radios.
You can imagine a remote control system to track the position, run the wench and limit travel, but the cost goes up. One possible solution that could be added to the 12 volt system, would be a wired or wireless camera positioned where it can see the wench and tracking system and connected to a PC or small monitor in the shack. Our prototype antenna goes from around 3.40 Megs to well about 4.4 Megs with the push of a button. We now believe the concept is sound. We believe the same system will work on 160 meters or any band where a single frequency antenna won’t  cover the entire band. As always, feedback is welcome.
Additional notes, suggestions and comments:
The coax feed comes up between the two pieces of Plexiglas and is held in place on one side only with zip ties. The ¼ 20 SS bolts are inserted through the “bottom”.  The 2 sheaves are dropped onto the bolts and the soldering is done before the “top” side is installed.

The “rules” are the same as for any inverted V. Nothing we’ve done has changed the operational characteristics of a “standard V”, as far as we can tell. What did change was the ability to find the best apex angle as it is now just a matter of tuning to any given frequency and looking at the data, then raising or lowering the antenna and comparing to fine the best center height relative to end heights.

Sam, KC4TAQ, the co-designer - lowered the tower from 55 feet (and antenna) to around 35 feet but made no changes to the end heights and found it moved the resonant frequency well down the band. All he had to do was take up on the wench (from in the shack) and it came back down to a 1 to 1 SWR. It does not appear that height is critical to this design.

We are using a DC window motor direct driving a wench spool with Dacron rope to do the adjusting. Due to the distance from the top block to the wench, the Dacron level winds perfectly onto the spool. As an alternative, you could use a small boat trailer wench or you may even find a large bait casting reel in a flea market if you want to change frequencies at the base of the tower. In this case, some means of tracking the frequency change will be needed. One option would be colored heat shrink tubing placed at proper intervals along the Dacron rope to create a visual scale.
When tuning from in the shack, we just use low power on the desired frequency and run the wench until the SWR falls to a minimum. Our objective was to design an antenna that would allow you to go to the local super center and find most of the parts you need.

As you see in the documentation, the critical thing is to keep the returning wire as close as possible to the top wire. Stiffer wire may work, but it hardens quicker than the flexible stuff but you should get several years of service even with #16 or 14 stranded copper.

The weight strips are to keep the blocks from flipping over. We didn’t want any extra metal in close proximity to the wire, hence the Plexiglas extension.
Extra weight (lead sinkers) can be added to the bottom of the Plexiglas strips if needed.

Theoretically, there should be no reason why you could not use this concept to go from 160 up to 40 meters or higher. It would require a multi-part pulley system at the turnaround block if you take up more than half of the wire in the system. A 40 meter trap may also allow for two bands. If you use a larger diameter wire from the feed point out to the trap then you should be able to cover the 40 meter band without a tuner.
The counter weights need only keep a reasonable amount of tension on the wires. We used about 12” of 3” PCV pipe with a cap on the bottom and have it about half full of steel punch outs from one of our iron workers. Sand bags would work equally well. We would like to hear from anyone that decides to build this antenna.
Designed, built and tested by W4QJP & KC4TAQ.

Simple principle of operation!
In the drawing above, when the wench is activated, it either pulls or releases the tension on the counter weights thru a system of pulleys at the apex and on the end of the antenna on both sides equally depending on whether you want to lengthen or shorten the two sides of the dipole. This is done using one rope connected about half way up from the wench where it connects to the two "control" ropes leading eventually to the end of each half of the dipole. By activating the wench, the total length of each half of the dipole is changed.
You simply watch your swr meter for the lowest swr on the frequency your tuning to!
When you reach that "sweat spot" stop! Your done!

Editors note: The designers report that about 6 adjustable feet on each half of the dipole is required for complete band coverage. (Longer on bottom end of the band, shorter on the top end.) Your total length results may be different due to your height above ground so some experimentation may be needed. But, once you get the correct total length for lowest swr over all portions of the band when remotely tuning the antenna, no more experimental length changing is needed. 

The key and most important parts of the system is the pulley mechanisms and the very flexible wire used as shown in the drawing below.

The end wire separation MUST be touching or kept at a minimum all the times.
As long as the "loop back" section is kept close to the upper wire, it becomes essentially  "invisible" to the overall length of the main antenna legs.
It is as simple as that!

Blue line is cord leading to counter weight. Upper left hand cord (not shown) leading to apex pulls
 is left out of drawing for clarity! (only one side of lower part of antenna shown)

Notice extra weight bar made from Plexiglass in lower right in drawing. This is used to help keep the pulley from "flipping over" by adding the appropriate weight hanging down from it.

Download a Pdf file of this project here!

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