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| Author |
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devnull
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mobile antennas
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Dec 17 00:32 UTC 2001 |
I've been thinking about what sort of antenna arrangement I might want
when I get around to putting antenna mounts on my 85 Suburban.
One theory is that perhaps I should install a set of 4 NMO mounts on the roof
in a configuration that would be suitable for mounting four identical antennas,
for direction finding, and then I could ordinarily have different antennas on
different mounts.
If I do that, I understand that none of the antennas in non-direction-finding
use will have the same radiation pattern that they would have if I only
had one antenna installed; the antenas will interfere with each other.
However, I'm getting the impression that while that may seem ugly in theory,
it may well not actually be a substantial problem in practice.
And it may be the case that for HF, I want something mounted on the back
bumper; that certainly would allow for a longer antenna, which ought to
improve efficiency, but then again, not having the ground plane of
the roof, I wonder if that cancels that out...
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| 53 responses total. |
gull
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response 1 of 53:
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Dec 18 01:03 UTC 2001 |
At HF the whole car's going to act like your groundplane no matter
where you mount the antenna, I think. I'd say the only disadvantage of
a bumper mount is the radiation pattern may be distorted a bit. But
again, on HF, a mobile whip is going to be so short as to be pretty
much isotropic anyway.
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krokus
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response 2 of 53:
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Dec 26 15:45 UTC 2001 |
Unless you're wanting the diamond pattern of antennas, you could just
put two nmo mounts up for df'ing. I'd say put them inline with the
heading of the vehicle, then use the typical tone-cancellation aspect.
Granted, you'll not be facing the way of the signals, but it'll be the
most useful for non-df use. (Of course you don't have to always use
all of your mounts, either.)
As for the bumper mount hf antenna, just be sure to use a good ground
to the antenna mount from the chasis.
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keesan
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response 3 of 53:
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Apr 12 01:34 UTC 2002 |
Regarding semimobile antennas. I was wondering why my clock radio gets
Lansing and Toledo better than an expensive receiver which is using as an
antenna a very long section of TV antenna wire (parallel wires in brown
plastic) hooked to one of those aluminum things with arms pointing in all four
directions. Jim was going to try putting the antenna up in the attic but when
he picked it up he noticed that the antenna was not connected to the antenna
wire. So apparently a long stretch of antenna wire is adequate - we
stretched it out along the house and noticed that attaching it to 10' of steel
channel improves the sound quality, if the sound channel is perpendicular to
a line drawn between the radio and the source. A new use for sound channel,
collecting sound.
Jim is hoping to come up with some system whereby we can have two
antennas, one optimized for Toledo and one for Lansing, and switch between
them with a switch. The Lansing direction works badly for Toledo because U
of M drowns everything else out with talk. Any ideas? He is thinking of
using a speaker switch box. Simpler than a TV antenna rotor.
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krokus
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response 4 of 53:
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Jun 7 21:49 UTC 2002 |
Using the speaker switch box will probably work, but keep in mind that
there will be an impedence issue there.
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keesan
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response 5 of 53:
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Nov 14 03:06 UTC 2002 |
We have a receiver which the neighbor fixed (it was losing one channel, he
replaced transistors) and when Jim attaches a rooftop antenna (aimed at
Toledo) it no longer gets the Canadian station at 89.9, instead it gets both
EMU (89.1) and some other unknown station. With a shorter antenna it gets
Canada (farther than EMU). The neighbor says he will attempt to adjust the
'front end' for greater selectivity, but we are wondering if there is
something we can do to filter out anything below 89.9. The neigbhor says to
use a bunch of capacitors somehow. How? Is there some way to set up an
antenna that only brings in stations between 89.9 and 91.3 FM? Does anyone
make an FM tuner for just the public radio band (of course we are still stuck
with EMU that way, and also UofM which drowns out Toledo unless you point the
antenna south).
With our antenna we can get Toledo and WKAR in stereo for the first time ever.
it is a nondirectional FM radio antenna but Jim pointed it at Toledo (tilted
it so that it is in a vertical instead of a horizontal plane, going east
west, and Toledo is roughly south.
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rcurl
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response 6 of 53:
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Nov 14 06:23 UTC 2002 |
Antennas themselves are tunable elements, so one can change the frequency
sensitivity by changing antenna length. This does depend upon the input
circuitry so it is hard to generalize. What kind of antenna is being
used? What is the input impedance of the receiver?
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gull
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response 7 of 53:
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Nov 14 15:45 UTC 2002 |
It's also possible to filter out specific stations with a 'tuned stub',
but I'd have to dig into my old college class notes to find the formula.
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rcurl
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response 8 of 53:
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Nov 14 17:16 UTC 2002 |
Your tuned stub needs to go into a matched transmission line (or the
formula is even worse.....).
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gull
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response 9 of 53:
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Nov 14 19:23 UTC 2002 |
True. I forgot they weren't using a tuned antenna as such. I used to
use the VHF part of an outdoor TV antenna (obsoleted by cable TV) as an
FM antenna. It worked *great*. Even cheap TV antennas make superior FM
radio antennas. I solved a static problem on the music-on-hold radio at
work by replacing the wire dipole it came with with an old set of TV
rabbit ears.
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keesan
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response 10 of 53:
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Nov 15 04:24 UTC 2002 |
We have one of those aluminum antennas consisting of two sort of flattened
loops crossed over each other as an X, with a long stretch of wire from that
to the 300 ohm screws on the radio (two parallel wires with plastic between
of the sort cheaper antennas are made of). If you detach the short piece
of that wire from the longer stretch leading to the roof, Canada comes in
properly. I wonder if we could fix things by making the wire an exact
multiple of 89.9 - what length would that be? The total wire is about 30'
at present. I think the FM band is about 5'? (Forgot all the terms).
The current solution is to detach the two pieces of wire in order to listen
to Canada and put them back for Toledo. Jim says this will all be obsolete
in a few years when radio goes digital and we will need to find a way to make
new tuners. (Or switch to internet radio).
Can someone help with calculations of the correct antenna length? I cannot
find my old ones. How to convert cycles per second to feet?
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rcurl
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response 11 of 53:
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Nov 15 06:16 UTC 2002 |
All you need to know is the speed of light, which is pretty close to a
nice convenient 300E6 meters/sec.
The parallel wire stuff is called "twin lead". Remember this stuff.
The antenna sounds like a "wide band" antenna - meant to NOT tune
sharply over the band.
I don't under what is "the short piece of that wire" if it is twinlead.
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gull
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response 12 of 53:
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Nov 15 15:25 UTC 2002 |
Supposedly radio is going to go digital in a way that is compatible with
existing analog sets, but I haven't seen details about this.
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keesan
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response 13 of 53:
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Nov 15 20:46 UTC 2002 |
The twin lead consists of short piece that was already attached to the radio
and a longer one that is attached to the antenna, and they are joined
together. Today I cannot even get Toledo with this setup, just Lansing.
The neighbor will try twiddling with the adjustments in the tuner section.
He will also see if he can figure out why my other receiver has stopped
producing the right channel in stereo but amplifies output to both speakers
in mono. That one tuned very nicely (in mono).
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n8nxf
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response 14 of 53:
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Nov 17 02:29 UTC 2002 |
Don't forget about the velocity factor when making tuning stubs out of
twinlead...
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gull
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response 15 of 53:
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Nov 18 15:56 UTC 2002 |
Yup. I forget what the value is, but I know it's somewhat higher than
for coax.
In practice, of course, you cut the stub a bit longer than the
calculated value, and trim it back until you get the attenuation you're
looking for.
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keesan
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response 16 of 53:
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Nov 19 00:40 UTC 2002 |
Are you saying we should take a piece of twin lead and adjust its length to
bring in 89.9, and then cut another one and adjust that for 91.3?
What is a velocity factor?
We left both receivers with the neighbor. I am now using a 1970s Lafayette
tuner, of which I have three (from Kiwanis because one channel was not working
which we fixed by cleaning switches, one from Reuse center for $10, one from
the curb, all needed new small bulbs and/or cords replaced). It has an
excellent tuner, 2 sets of speakers, tape, aux and phono (magnetic or ceramic
switchable), fm mute, loudness, hi filter (what is that for?), mpx filter
(ditto?), but no fancy meters or presets or quartz tuning. How does a quartz
tuner work? (This is the radio conference, right?). This info should be
relevant for adjusting the bad one.
I am getting stereo reception that sounds good on this receiver with the
rooftop antenna, from Toledo (our weakest station)! The antenna is pointed
at Toledo.
I have another Lafayette that is older, no tape input, aux input dead, so I
plug the CD-ROM drive into the ceramic phono imput. What might make an aux
input dead?
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gull
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response 17 of 53:
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Nov 19 15:05 UTC 2002 |
No, a tuned stub filters out a specific frequency. So if you have
interference from a nearby station, you can trim the stub until that
station is nulled out. It's connected to the same terminals as the
piece going to the antenna, but it's left unconnected on the other end,
or the end is shorted (depending on the type of stub).
If changing the length of the feedline to your antenna makes a
difference (which it does, if I'm reading correctly) then your antenna
probably isn't matched very well to the feedline, and you're changing
the overall match by changing the length of the line.
Velocity factor is how fast a signal travels along the line. It's
expressed as a fraction of c, the speed of light in free space. 75% is
a typical factor for coaxial cable, meaning the signal travels at 0.75c.
A quartz tuner is usually one that synthesizes the local oscillator
frequency needed to tune in the signal from fixed frequencies generated
by one or more quartz crystals. This is different from a "regular"
tuner, in which you vary the local oscillator frequency by adjusting a
variable capacitor in the circuit. Tuners that use frequency synthesis
tend to drift less, and allow for digital tuning, but they involve
intermediate frequencies that have to be filtered out to avoid
interference problems.
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keesan
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response 18 of 53:
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Nov 19 22:29 UTC 2002 |
So perhaps the filter is not filtering out the intermediate frequencies?
We are getting two unwanted frequencies at the same time instead of one that
we want, if the signal is strong. The problem is bad even without the rooftop
antenna (with just a wall T-shaped antenna, or just a short piece of
twin-lead), but worse with a good antenna. The same antenna and lead on
another receiver work perfectly so that is not causing the problem. When we
detach the antenna from the shorter half of the lead, sometimes we get the
right station (weakly, in mono only).
How would you go about adjusting this quartz tuned receiver so as not to tune
in 89.1 and some other station at the same time, when set to 89.9? Do we
stick two short pieces of twinlead on the antenna screws and trim them until
each unwanted station goes away? Do we stick two more on to get rid of the
two stations that are coming in instead of Toledo (91.3)? Our neighbor was
going to try fiddling with the adjustments in the 'front end'. Not a whole
lot to lose.
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gull
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response 19 of 53:
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Nov 19 23:35 UTC 2002 |
Yeah, it sounds like it's definately malfunctioning somehow. It's hard
to say what the problem is likely to be, but poor "alignment" of the IF
stages could be the culprit. The thing is, it's hard for that to
happen unless someone tweaks the adjustments in an attempt to "improve"
things. There's a very specific set of steps and specialized tools
required to adjust it properly, and attempts to do it by guesswork are
almost always disasterous.
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keesan
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response 20 of 53:
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Nov 20 03:08 UTC 2002 |
I watched someone readjust another receiver by ear. He was very experienced
and it came out perfect. But the problem was different, just that the whole
thing had drifted, not that the selectivity was so abysmal, and it was not
a quartz tuner. As I said, there is nothing to lose on this one as it is not
usable as a tuner (plus if radio switches to digital in a few years it would
be useless anyway). Gull, want to give this a try?
Our neighbor guessed a bad transistor. He fixed another problem with the same
receiver that was two bad transistors (one channel intermittently dead).
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gull
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response 21 of 53:
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Nov 20 03:47 UTC 2002 |
I'm not really interested -- I've been trying to cut down on how much
junk I have lying around, so I'm only taking on repair jobs on items I
intend to actually use, these days. A one-bedroom apartment fills up
pretty fast.
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keesan
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response 22 of 53:
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Nov 21 03:01 UTC 2002 |
I know, I live in one that is full of fixed or to-be-fixed tape decks,
receivers, computers, etc.
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keesan
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response 23 of 53:
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Nov 21 20:36 UTC 2002 |
I am told it is the IF transformer that is going bad or is out of adjustment
and you need a special tool to adjust it. It is not properly filtering out
the unwanted signals.
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rcurl
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response 24 of 53:
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Nov 21 21:56 UTC 2002 |
There are usually two or more IF stages. Getting them all balanced
correctly requires instrumentation.
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