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| Dear Ed,
First, thank you for taking the net so graciously last night. I just hadn't
prepared as I usually do, and so was nervous about going ahead. I know I
learned a little from you about how to handle hard-to-hear call signs. I did
feel bad about your pen, glasses, etc. Sorry! I'll be ready this coming
Sunday, unless Cathy would like to do it.
Also, would your article here on 6m be a good one for the newsletter? There are
over 70 people who are on the newsletter list, but not on the CVARC listserv,
so it would be new to most of them.Will Sasha bein Austria on the night of the
CVARC Holiday party dinner? If so, would you like to ride with KB1NBP and me?
No need to answer right away, but perhaps keep that possibility open.
All the best,J
Jessica Noyes KB1NBO
P.O. Box 112, Marshfield, VT 05658Skype ID j_yesno Phone 802 426 3671
www.proz.com/profile/828566 CallSend SMSAdd to SkypeYou'll need Skype Credit
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From: Ed Hutchinson <ehutch@xxxxxxxxx>
To: cvarc@xxxxxxxxxxxxx
Sent: Monday, November 20, 2017 4:40 PM
Subject: [CVARC] Re: 6 meter test net
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