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...

53 responses total.

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.

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.

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.

Using the speaker switch box will probably work, but keep in mind that
there will be an impedence issue there.

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.

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?

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.

Your tuned stub needs to go into a matched transmission line (or the
formula is even worse.....). 

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.

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?

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.

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.

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).

Don't forget about the velocity factor when making tuning stubs out of
twinlead...

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.

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?  

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.

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.

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.

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).

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.

I know, I live in one that is full of fixed or to-be-fixed tape decks,
receivers, computers, etc.  

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.

There are usually two or more IF stages. Getting them all balanced
correctly requires instrumentation.

What sort of instrument do you suggest?  I asked the neighbor to be careful
to mark the current positions of anything that he adjusted, before adjusting.
Kiwanis currently has several tuners for sale but why do things the easy way.
When is radio going digital only?  Time heals all.

This is just a guess, not know what type stuff you have, but in general
you'd need an RF signal generator and an analog volt meter.

The hard part if you don't have schematics is finding right tests points
to measure, and which tuning capactors or inductors to adjust.

Also just marking the positions on something like a tuning slug or cap
is not always a garantee that you'll get it back into proper alignment
if you happen to mess up...some are very very sensitive, and certain
tunable components don't adjust with only 1 turn...I've had some tunable
cavities that you have to screw in the tuning thingy like 100 times
before it will either stop physically or get to the position you need 
it to be..

Consult an ARRL Handbook for general alignment instructions. 

Marking positions for the alignment screws will not be very useful. The
adjustments are *very* critical, as KC8BYL says, and what one has done can
only be seen clearly on instruments. For example, there is hysteresis in
the adjustments, so one seems to overadjust because settings settle back a
little, and this source of error is not reproducible because of variable
friction. One also has to align stage by stage, so must measure the
alignment between stages, which requires picking up the signal within the
circuitry. You can't align properly by just listening to the radio.

Someone with a lot of experience did align my other receiver, by ear, so it
would tune in all the stations I listen to.  It works perfectly now.  I listen
to the public radio stations, and the other end of the dial may not work but
I don't care.  

Do you really need an RF signal generator instead of just picking up a radio
signal from the air?  I am sure the neighbor has a volt meter.  [B
We have an RF signal generator somewhere which someone at Purchase Radio
(since retired, used to teach electronics repair) fixed after Jim
unnecessarily bought capacitors there (he took them back out).  Jim says yes
you need it to get a specific frequency to adjust with.

Anyone want to come over and help adjust the tuner?  Three people might be
enough (I can always help by plugging things in an out).  

We do not have schematics.  Someone really nice at a company that sold
transistors once got hold of the schematics for a Yahama receiver for us. 
We kept replacing a transistor and it kept burning out.  The neighbor across
the street who travels around the world fixing expensive tape recorders for
Sony came over and pointed out that there was a line missing in the schematic,
after which we tracked down a bad solder joint.  Took two years, taking
transistors etc. in and out, measuring everything.  Nice receiver.  It was
by the curb, dead.  My current receiver (Lafayette) was by the curb and works
perfectly except for a few burnt-out lights.

Back to the problem of the disappearing channel.  I replaced the Pioneer with
no right channel (from the curb) by the Lafayette (from the curb) which had
intermittently no left channel, then got to the point where it had left
channel for only a few seconds after it was turned on.  Tried twiddling the
VOL/BALANCE knob in case that was corroded.  Mono produces sound to both
speakers so the amplifiers are okay - same as in the Pioneer.  Phoned the
neighbor and asked what else to try.  He suggested trying to clean the tape
monitor switch, so I pushed the button in and out about ten times and started
to get sound, which sounded bad and cut out a lot but was on the right track.
Another 50 or so pushes and I am back to stereo sound.  I never would have
guessed to try this as a fix.  He is still working on the tuner problem, but
in a few years nobody will be broadcasting analog anyway.  If we got some sort
of adaptor for digital radio (which I read was going to start being broadcast
in some places this January) would any of the 'front end' of the original
tuner still be used?  The receiver sounds really nice, says the neighbor who
fixed the missing right channel (in this case it was bad amplifier
transistors, $10 in parts).

What sort of antenna do you need to pick up digitally broadcast radio?

Question:  Which is likely to perform better on 10 meters?  A 4' long
base-loaded whip mounted on the rain gutter, or a full-size, unloaded
quarter-wave whip mounted on the bumper?  My gut feeling is that the
unloaded whip will have better performance because of lower losses, but
I'm wondering if mounting it lower will cancel that out.  (Obviously I
can't mount an 8' unloaded whip on the rain gutter of a 6'11" van and
still clear bridges and power lines!)

Probably  a bigger issue is the radiation pattern (and no, I don't know which
is "better", which depends on how  you define that).

I imagine when it comes to the radiation pattern, both are likely to be
"cloud warmers".  We *are* talking about a 1/4 wave antenna close to the
ground.  To get a really good low-angle radiation pattern would take a
5/8-wave whip, and that's just too tall.


So I don't think there'll be large differences in the pattern, though
the bumper mount unloaded whip might show a null in the direction of the
van body.  The unloaded antenna will have lower resistive losses, but
I'm not sure if those will be cancelled out by the losses caused by half
of its length being close to the steel body of the van.

Don't forget that you're less likely to do damage to things with the
lower mounted antenna, especially since you can make/use a sturdier
mounting arrangement.  You can get a better ground, although this won't
give you the normal increases.

How about making a horizontal array along the roof?  :)

The roof is fiberglass, so mounting an antenna on the roof is out of the
question.  The highest metallic point on the van is the rain gutter.

I may go ahead and try a full-length quarter wave whip mounted on the
bumper.  The whip can be had from Radio Shack for only $10, and a bumper
mount won't cost me more than twice that.  The rear bumper is already
smashed so drilling holes in it won't affect its value any.  In close
clearance situations, like forest roads, I can bend the whip over and
tie the tip to a point on the rain gutter.  If I don't like the
performance, I can always cut it up and make a 2-meter band dipole for
my apartment out of the pieces. ;)

If the roof is fiberglass, could you put the antenna inside the roof?

Doesn't that base loaded whip have to still have a ground plane? The
metal gutter alone would't seem to be sufficient for that. 

Having a fiberglass roof does potentially present a problem for RF
exposure.  How much power are you looking at transmitting with?

Re #36: It wouldn't, but the gutter is attached to the rest of the van
body, which is steel.

Re #37: 25 watts.

That might detune it as then the load coil is not at the (ground) "base"
of the antenna, as it would be designed to be. Anyway, I would suggest
you test the SWR for mounting it that way.

- Backtalk version 1.3.30 - Copyright 1996-2006, Jan Wolter and Steve Weiss