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ASK BOB
GENERAL QUESTIONS RELATED TO RADIO
Bob Grove, W8JHD
bobgrove@monitoringtimes.com
Q. I’m going to put up an inverted
V wire antenna. I have looked for
stand-off insulators for the 450
ladder line which will run down the
telescoping mast support but have
not been able to find any on the
market. Do you know of any source
for commercially made insulators
of that sort? How far from the mast
does the open-feeder ladder line
have to be kept? (Gary Britten,
W4GNB, Wilmore, KY)
A. I haven’t seen any since most folks are now
using coax cable leading up to resonant antennas,
and HF dipoles are usually end supported
and center fed so that a standoff isn’t necessary.
So far as the inverted V, the line usually tapers
from the feed point down to the shack; it isn’t
fastened to the center mast.
Remember those TV standoff insulators that
ran down the mast for 300 ohm ribbon line? I
wonder if a few of those would work, simply running
the ladder-line spacers through their round
ends. You’d probably have to cement or clamp
the intersection of the spreader insulator where
it runs through the round end of the standoff;
otherwise, during windy weather, the ladder line
would keep slipping back and forth, allowing the
open wire to touch the metal and be grounded or
at least electrically unbalanced.
The spacing of open line from large metal
surfaces is good at just a few inches, not even a
foot. I’d go with 4 inches minimum.
Q. My antenna analyzer shows a
bad impedance match (high SWR)
between my horizontal HF loop
antenna and my rig. I calculated
the loop to be 254 feet long at 3.9
MHz using insulated wire. What
could be wrong? (Rick - KF5LSN)
A. Your original calculation was very close.
Dividing 1005 by the lowest frequency in megahertz
gives the correct circumference in feet.
Since you are using insulated wire, you should
reduce the circumference by 4%. This would be
247 feet.
The feedpoint impedance of such a loop
(or a dipole) is about 100 ohms, a 2:1 SWR if
you’re using coax feed. In some cases, changing
the length of the transmission line is as important
as the correct antenna length.
It’s a good idea to use open-wire feeders
(twin lead or ladder line) for antennas with a
high or variable feedpoint impedance, especially
multiband antennas. Because of the separation
of the conductors and the amount of insulation
between them, there’s almost no loss whatsoever
under high SWR. The tuner adjusts the impedance
mismatch between the antenna system and
the rig.
Here’s a good article on HF loops: www.
k5rcd.org/hor%20loop%20instruct.htm
Q. If the PAR End Fedz antenna
is properly tuned, can it be used
for transmitting in the 10-40m
bands? I autotuned it the other
night on 7.200 MHz and came
up with a respectable SWR. (Matt,
email)
A. Autotuners work well within certain SWR
limits, but if it’s excessive, then you will need a
manually tuned transmatch. Still, keep in mind
that the mismatch is still present between the
coax and the antenna feedpoint; the properly
adjusted match is made only at the transmitter/
tuner junction.
Since the PAR End Fedz is designed for
receiving only, there may be some components
within the insulated housing that would be
vulnerable to excessive impedance mismatch as
well as proper RF current and thus be damaged
by the heat.
You should check with the manufacturer
for their recommendations regarding maximum
RF power and SWR vulnerability. You are
probably all right at low power (10-20 watts),
but check regularly to see if you can feel heat
being generated in the housing.
Q. I was wondering about the
differences between ceramic
and monolithic capacitors. Do
they function differently? Are certain
circuits or functions better
suited for one or the other? (Van
Wilshire, email)
A. Ceramic merely refers to the insulating
material as compared to paper, mica, polyester,
resin, or Mylar for example. Monolithic simply
means that several layers of the insulation are
alternately interleaved with layers of the metal.
Then the whole block is compressed into a
monolith (single chunk) as compared to being
rolled into a cylinder (tubular).
Yes, different styles work better at different
frequencies. An excellent illustration of these
differences and their applications can be found
at www.gqrp.com/na5n.pdf.
Q. My new digital scanner is not
picking up signals as well as my
old analog scanner. What sort of
antenna should I put up? (Gregg
Fear, email)
A. Can you hear analog signals as well on your
new scanner as you did with the old scanner? In
a side-by-side comparison there should be no
difference in reception since you are listening
to the same signals. If there is, then I’d suspect
the scanner.
Are you having to use a different adapter on
your antenna cable than you were before? Make
sure it’s not loose and is making a good contact
with the scanner receptacle.
Digital signals must be stronger than analog
to be heard properly. As with TV, analog signals
can tolerate some static and still be seen, but
digital needs all the pulses and no noise to get a
TV picture and sound.
For scanner coax, don’t use RG-58/U in
long runs; it’s very lossy at VHF and especially
UHF. Use RG-6/U outdoor TV coax or, in the
worst cases, more expensive low-loss cables like
RG-8/U, Belden 9913, or LMR400.
The lowest cost, high performance scanner
antennas include the Grove Scanner Beam III
(which is directional) and the ScanTenna (omnidirectional).
You can see their specifications
at www.grove-ent.com/scannerbeam3.html
and www.grove-ent.com/ANT7.html.
Q. My new floor lamp takes standard
light bulbs, 60 watts maximum.
But the instructions say not
to use CFLs exceeding 13 watts!
CFLs are known for operating
much cooler than incandescent
and I’d like to use some bright 30
watt CFL’s. (Judy May, W1ORO,
Union, Kentucky)
A. There’s nothing wrong with putting in those
30 watt CFLs; they will operate cooler than the
60 watt incandescents, and provide brighter light
as well!
(Note: Judy let us know that she went ahead
and tried the 30 watt CFL, and then a 42 watt
CFL, and both stayed cooler than the 60 watt
incandescent!)
Questions or tips sent to Ask Bob, c/o MT are
printed in this column as space permits. Mail
your questions along with a self-addressed
stamped envelope in care of MT, or e-mail
to bobgrove@monitoringtimes.com. (Please
include your name and address.)
August 2012 MONITORING TIMES 23