[CVARC] Re: 6 meter test net

  • From: Ed Hutchinson <ehutch@xxxxxxxxx>
  • To: cvarc@xxxxxxxxxxxxx
  • Date: Mon, 20 Nov 2017 16:40:07 -0500

I have made a few 6m dipoles... It is quite easy....

There are lots of reference sources, but if you want to build one and don't want to bother to look up a how-to.... You can follow the procedures below...

I usually use 1/2" PVC about 4 inches long for the center insulator. If I am making it "pretty" I mount a chassis UHF connector stuck into a 5/8" hole in the middle one side, and glued, or zip tied through the holes in the connector (with soldered lead wires extending to beyond the ends of the insulator).  Or just solder coax to the antenna wires, but the center conductor joint will be brittle and will break someday.

For the outer ends I often just make a U in the wire, with the excess wrapped back toward the center, and I simply tie plastic line to the U (with an anchor bend).   That makes adjustment easy. I cut off the excess and solder the U when tuning is done.  The plastic line will work very nicely as long as it stays dry. If I finish with PVC end insulators,  I only go through one "side" of the tube with 6M dipoles to keep the loop small.

Start at the "target" frequency in free space

I use an on line calculator because I'm too lazy to look up the speed of light:

http://www.procato.com/calculator-wavelength-frequency/

Theoretically "half wavelength" is 117.511 inches total length at 50.220 MHz.

I do have a small 5M metric tape measure and I should use cm, but I have habits...

That would be 9'  6 1/8" to the ends of the wires at the end insulators.

The length of the center insulator will need to be counted as length.

So each side would be 58.756 inches minus half the length of the center insulator.

However,  this will actually be a little bit too long, because of "end effect", the termination to the coax, and wire diameter. Things in the near field will also skew the tuning, so a measurement that works when raised near my barn may not work off your tower... Measure and prune!

Correction at 50.220 is about 2 5/16 inches per MHz. So test and prune....

I usually cut the element sides a couple inches too long (for 6m, but three or four feet at 1.9MHz), and only solder the center so I can prune the outer ends by raising the antenna and measure the resonance (with an MFJ-269 analyzer, but listening with a receiver because the meter's frequency is not accurate) .

To be really precise I tune through a feed-line that is a multiple of an electrical half wavelength (any number of half-wavelengths times the velocity factor of the cable used).  A lab grade analyzer would also help but I don't have one.

By the calculator, 1 wavelength is 18.89 feet, and the electrical wavelength of RG213 with a velocity factor of .66 will be 12.04 feet.  So if you need about 45 or 50 feet of feed-line to tune the antenna while raised, then 4 electrical wavelengths is pretty close to 48 feet.

The MFJ-269 can measure an electrical line length to get to any number of half wavelengths. Use a tee connector and a 50 Ohm resistive load. If you need 4 wavelengths, that is 2 wavelengths at half the frequency, 1 wavelength at 1/4 of the frequency,  a half wavelength at 1/8 the frequency, and a quarter wavelength at 1/16 the frequency (calculation is easier with 4 wavelengths than it is with 5).

If the 4 wavelength coax (open stub) is connected to the tee, with a dummy load, the SWR should be 1:1 at 25.11 MHz, 12.55 MHz, and 6.278 MHz . At those frequencies the coax will reflect a very high impedance at the open end back to the tee, and the meter will see only the dummy load.  If the end of the coax is shorted it will be a shorted stub 1/4 wave stub at 3.139 MHz and should also have 1:1 SWR at that frequency, because  the shorted end will reflect a very high impedance back to the tee at 1/4 wavelength. The sharpest indication is at 1/4 wavelength so I start there, and for some reason it is not always "linear" so tweek to the higher frequencies.  This will get very close except for the length of the tee connections and the asymmetry of the short.  I start with the coax a few inches long, measure and prune, then add the other end connector, if using a connector at the antenna.  An inch or three is probably close enough over 4 wavelengths.

To tune the antenna I use a pulley on at least one end, or the center, and enough line at the other end(s) to be able to reach the ends of the wires when the pulley end (or center) is dropped. Since a "flat-top" dipole has an ideal impedance of about 73 ohms, it is likely that something under 2:1 SWR 73 ohms resistive and 0 ohms reactive, is the about the best possible tuning.  Feeding the dipole with 70 ohm (TV) coax is probably better than 50 ohm coax, ignoring the mismatch at the transmitter or fixing it with a tuner at the transmitter end.  Or, sloping the ends down to an inverted Vee will correct the antenna to 50 ohms, that will usually also lower the resonant frequency a bit.  I like inverted vees.

"Murphy" declares that it is almost certain that if the first pruning is 100% of the difference predicted it will end up too short.

I measure the resonance, let the antenna down and prune only one half of the difference predicted (one quarter each side).

Usually a second pruning pass will just about nail the target frequency.

The closer to actual resonance, and the better the match to the feed-line the better the antenna will work.  At VHF any standing waves in the feed-line may "tune out" with a tuner but they will consume power as resistive loss in the coax. The line loss is proportional to the number of wavelengths of feed-line, which can be quite a few at VHF.

Ed


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