In a message dated 9/15/01 10:41:28 AM Pacific Daylight Time,
Has anyone ever had the "resonant" dip on their TH6 appear way down on 27.6
Mhz or lower?
I have a TH6 @ 42' and the SWR's low points (1:1) are
20.870MHz , 14.100MHz & 27.6MHz
On 15m/20m it's not too much of a problem as the bandwidth is wide enough to
pretty much cover me on the CW and LO-Phone portions of the band, but on
10m it's too low and the dip is quite sharp
Seems it's set a wee lower than it should be in freq for 10m. There's also a
dip on 28.9MHz (2.1:1)
My real question is whether I can change anything by removing only the
driven element. Taking the entire beam down is pretty much out of the
question. I "think" I might be able to remove only the driven. (anyone
I am feeding it with about 75' of 3/4" 75Ohm CATV line.
Michael, Coreen & Corey Smith
(VE9AA, VE9AAA & Baby-VE9)
271 Smith Rd
Waterville, NB E2V 3V6 Canada
A SWR dip at the end of the end of a random length of coax can have some
variables in it in particular when using 75 ohm coax and you didn't mention
what the Zo of the bridge was designed for. You didn't say if it was ever
checked with 50 ohm coax and bridge and if a time period has elapsed since
new. Using multiples my "Magic Length" of 91' 2" of .66 VF coax has solved
some SWR problems for some. Try different bridges.
I'd disconnect the coax and balun and grid dip it at the hair pin short. You
may get the DE resonant frequency and weaker versions that of the reflector
and director. It's been awhile since I've done this.
I'd take the DE off and do a complete overhaul on it. GD the traps and see
if they are balanced real close, record the data and reinstall. I do this on
a new trap beam for "future reference"--like now. Sometimes a clean job will
solve a problem. Did you use a conductive grease? Often traps in the DE are
actually resonant lower than the band and the stubs in-between reduced in
length to retain resonance. The reason for this is aluminum wire was used in
the trap and if resonant in the band the circulating currents were too high
and it heated.
Do the SWR test with new 50 ohm coax, a balun and a 50 ohm bridge. Shorten
the DE length to the 10M trap to get what you want for resonance but you will
have to lengthen it a bit on 15&20M to compensate.
Depending how long it's been up I'd take the whole beam down and disassemble
the traps, clean every joint and regrease. Most hams are under the total
allusion that joints remain the same after a time. I check them with a HP
low resistance ohmmeter. A conventional ohmmeter is often all you needed.
Sometimes you need a Capacitance Meter. After you clean a few beams you will
find that they don't stay the same and many yagi's up there are not up to
snuff. I've kept careful data of many yagi's and trap verticals and was able
to detect problems in change of the lowest SWR, value, frequency or
comparison to a reference antenna or rerun the patterns on a pattern recorder
with my pick up dipole and then corrected the problem. I got what I called
the "K7GCO Yagi Saw Tooth Performance & Efficiency Curve." "Yagi drop off"
is slow and gradual and unless there is a noticeable SWR change or F/B drop
off, many aren't aware of any change in performance UNLESS they have a
properly made quad reference beam. I usually cleaned the joints with special
brushes before a test or contest. Every fraction of a dB helps in a contest.
There is one big misconception with yagi's. They work great when brand new
and unless properly greased they can drop off. You think that you got the
ham world by the "RF Tail." Your "Signal Strength Ego" is elevated to great
heights. That's all some live on. The technical problem is to keep it up
there. Then over a period of time you get RF'd out by others more
often--sometimes just with a new yagi or even a quad and you attribute it to
skip and "other rationalizations."
In the May 1925 QST that sold for 25 cents there is 46 pages of antenna
material that reads like the Antenna Handbook everyone should read. Get it
from the Library. Those old QST's have great material and I've read them
all. It even has some "would you believe humor." They had great "RF
Insights" back then which you need to be competitive. A lot of coal was
burned then and the soot covered the insulators and corroded the wire. They
noticed the receive would drop off--even without a S-Meter. They had great
"Ear Meters." They cleaned the insulators and changed the wire if not
insulated and they were back in the maximum signal strength business. So
this concept has been around for a long time. I'm a couple miles from salt
water in Seattle and it's a problem. I started this cleaning procedure in SD
back in the 30's and we didn't have a salt water problem. There is perhaps
less soot from oil now than from coal burning. I have a heat pump and
electric heat 4 miles out in the country and with buried power lines at my
new 6 acre QTH in SD with very high ground conductivity according to the
Government maps I have.
With a quad and "one soldered joint per element" the joint resistance STAYS
THE SAME year after year if properly done with regular solder. Silver solder
is not needed. Eutetic 1020 SS is great stuff I use on aluminum joints etc
along with very inexpensive No Flux Flee Market Aluminum Solder and only a
BurnsOMatic torch is needed. The Performance & Efficiency Curve of a quad is
horizontal and above a yagi. The "Quad No Maintenance Concept" allowed me to
spend time on other antennas.
Another concept has been reported to me from several. They put up a properly
made, tuned and matched 2&3 element quads for a "reference beam" for their 3
or more element yagi as I suggested and a couple of years later took down the
yagi. Isn't that interesting? Can you "read between the elements?"
To really check out a yagi on the range before Eznec it needed to be scaled
to say 146/450MHz which is easy to do mathematically. It's difficult to do
mechanically as model air plane brass tubing comes in only certain wall
thickness etc. It's virtually impossible to scale yagi elements unless is
used to obtain exact diameters and that's too time consuming even with a
small diameter rod follower in the lathe. Copper wire is available in so
many steps you can scale a quad easily for wire diameter. I often use the BC
FM band of 88-106 MHz for scaling and use all those 24 hour RF sources at
different directions. You can use a FM receiver with an S-Meter for a FS
Meter. There is a Ch2 TV station 90 miles away I'll be using as a RF source
also. I have a JFD FS Meter that covers all these frequencies and a pattern
recorder I made with selsyns. k7gco
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