Conference vmszoo::rc

    If your antenna is longer than your vehicle you are going to have
    to wrap it up anyway, so a base-loaded antenna should work about
    as well.  Cutting the antenna wire shouldn't be a problem, just
    measure it ahead of time and if you want to return to the original,
    make sure it's the same length.
    
    Plastic insulation is invisible to radio waves, so that won't make
    any difference.  If you want shielding, put in a piece of mini coax
    like RG-174, though it _may_ detune your reciever a bit....
    
    Willie

    I don't think you really want to sheild your antenna. if you sheild it
    from electrical noise such as servo motors etc, you probably are
    sheilding it from the desired signal as well. Simply try to avoid
    running it right next to such sources of interference. Also if you
    change to one of those base loaded antennas, you possibly will need to
    retune your receiver. I simply run my regular receiver antenna through
    a chunk of that yellow nylon pushrod stuff.
    
    Wayne

I personally don't think that the loaded antenna will make much
difference in the receiver.  What is critical is the tuning of
the antenna coils (more on that later).  If your wire antenna is
cut to 1/4 wavelength, and the coils are tuned correctly, you
should in theory get no difference in performance (might even be
less, since the loaded antenna is shorter).

This assumes that the loaded antenna is set for 1/4 wavelength;
it would be foolish for the manufacturer to do anything else,
since it would require that you retune the receiver, something
that most folks cannot do. The hitch would come if your wire
antenna is other than 1/4 wavelength, and the receiver is tuned
for that -- then the loaded antenna would hurt your performance. 

However, I'm skeptical that it would make any difference in any
event, since the currents in the circuit are infintesimal (sp).
Loading is significant in transmitting antennas.  But I could be
all wet on that point.

I had to go back into my silver seven receiver last week to fix
the pc board.  I had a meeting with the turf last winter that
knocked my receiver out.  I went in and found what I thought the
trouble; a bad solder joint in the first IF amp.  It happened
again last week, and very intent on getting to the cause, I went
back in.  I don't like it when I "fix" something and its still
broken!

Anyway I got news for anyone building the S7 rx: be sure to
solder the tabs on the IF cans to the pads securely.  It seems
that Fred Marks, in his infinite wisdom uses the shell of the IF
can to carry voltage to the IF amplifers!!!  Had a very difficult
to find cold solder joint on one of the tabs.  If you want to get
motivated, I'll tell you that when the joint opens up the rx goes
dead and the servos are in whatever position they're in.  If the
plane is going straight and level, wave bye-bye.

In retuning the receiver, I decided to be creative and follow
directions.  I used an oscilloscope on the detector output and
tuned for a peak, like I've done before.  The difference is that
now I removed the antenna on the Tx and peaked the RF coils up so
fine!  With the antenna removed and the tx placed about 30 feet
away in another room, I get a nice signal on the scope, and each
coil and IF can has a very distinct peak.  Its hotter than a
two-dollar pistol.

Maybe I should follow directions more often.

    To be more specific, assume your antenna is a quarter wave [receiver
    antennas are not nearly as critical as transmitter antennas anyway]
    and you want to shield it for 6 inches as it runs by the servos,
    do the following:
    
    Open the receiver, remove the antenna wire, and solder in a piece
    of RG174 (or the 75 ohm stuff, the number of which escapes me at
    the moment....), connect the shield to ground inside the receiver,
    and solder the 1/4 wave wire antenna to the other end of the coax,
    leaving the ground open at that end.
    
    This may help, but it's even money wether the results will be
    noticable, and if it will improve matters or not.....
    
    Willie

    just a brief comment....  The center conductor in typical coaxial
    cable is almost impossible to get a good solder connection to. 
    Shielded wire like that typically found in stereo component
    interconnects will probably work as well as coax, and be much easier
    to deal with, since it is much more pliant.  You must have a good
    ground point for the shield or it won't do any good.  This point
    ought to be as close to where negative voltage hits the Rx from
    the battery, otherwise you may end up just coupling all that electrical
    noise the shield catches to the receiver circuits through the negative
    supply rail.  I Don't think that 75ohm  wire should be used.  The
    standard receiver antenna wire has no impedance (it's just wire)
    and putting 75 ohm impedance wire in the antenna circuit will detune
    your reciever.                                      

Yes, but I don't think that putting audio cable in there will
help.  Maybe it would provide some measure of shielding, but
it would not be of "invisible" length to the receiver front end.
You would have to cut the antenna wire by a corresponding length.

But no matter what you put in there, the receiver input coils
should be retuned, since as was pointed out, the coils are
optomized for the dangling wire.  Looks like careful routing is
all you can do. 

Next question: how many angels can dance on the head of a pin?

    
    Just a small point re. -.2 soldering to the center conductor 
    of just about any piece of coax should present no problem at 
    all provided you turn the soldering iron on.
    
    Mike
    

    I have always had a hard time soldering to typical TV type coaxial
    cable, the center conductor is not copper (but something much harder)
    and solder simply does not take well to it.  Why ?  I DON'T KNOW!

    That is because that cable is 300 Ohm cable and the center conductor
    is made to be used as the center pin of the conductor and cannot
    take solder, whereas RG 174/56 or almost any 50/75 Ohm coax usualy
    is copper or some metal that takes solder i.e. nickel plated.
    
    Mike
    

    Not that I wan't a fight, or anything, and I am certainly no expert
    in matters pertaining to analog electronics, particularly those
    aspects involving RF type stuff, but I beg to differ.  MOST, if
    not ALL, coaxial cable for TV use is 75 Ohm impedance.  Twin-Lead
    is the antenna connection of choice when 300 Ohm is desired.  In
    fact, both my TV and VCR and stereo tuner came with a little matching
    transformer to change 300 Ohm twin-lead into 75 Ohm Coaxial.  Also,
    I have purchased a little thingy from Radio Shack that changes 75
    Ohm coaxial into 300 Ohm twin-lead, since my old TV did not have
    a coax input.
    
    I will say no more on this subject, since this is not really the
    place to discuss it, and I hate to be an annoyance.  If I'm wrong,
    I accept defeat.  If I'm right, I hope I've been of some help to
    someone.

    A couple of points:
    
    The hard center conductor of some TV coax is aluminum, and there's
    virtually no way to solder to it.  However, most other 'normal'
    coax has a copper center-conductor and shouldn't be any problem
    to solder to.
    
    The impedance of a piece of wire isn't nothing at all, it's about
    75 ohms, assuming a quarter-wave antenna.  This is why most
    communications gear uses 50 or 75 ohm coax for connecting antennas.
    I believe you get 75 ohms with no ground plane and 50 ohms with
    a ground plane, but don't quote me.
    
    Assuming a 75 ohm impedance, you can extend your antenna wire with
    75 ohm coax without having any impact in the receiver (ideally).
    Mismatched impedances (such as you would have with audio type shielded
    cable) are more likely to cause problems.
    
    RG-174 is very small compared to RG-58 or most CATV coax, and would
    be far more appropriate for modelling.  Note that this will only
    make a difference _IF_ the reciever is picking up interference through
    the antenna wire and not thru inductive pickup or power lines or
    lots of other places....
    
    Willie

    This past weekend I somehow pulled the antenna wire out of the
    reciever.  It actually broke off about 1" from the solder joint.
    
    My question is....Can I just discard the 1" piece of wire and re-
    solder the wire to the receiver without screwing up the range?
    
    What effect will a shorter receiver wire have ....lets say 2" shorter
    than normal?
    
    
    Ken

You'd be better off replacing the wire with one piece the exact original length 
to maintain your tuning. Do a range check in a familiar field (where you've 
done one before. You have done one ;^) in a familiar plane and compare the 
results. Best thing would be to send it in to be retuned during the slow season 
(I hate to think of winter coming ;^)

one inch make a big difference from what I saw when I was doing the same thing 
with my old Heathkits. The wire length SHOULD be some simple factor of the 
wavelength but who knows how strict the commercial manufacturers are. 7/19ths 
isn't a very SIMPLE factor ;^)

    Ken,
    
    I've successfully avoided anymore technical/theoretical involvement in
    out radio gear than is absolutely necessary to operate, care and feed
    it.  However, I know Jim is correct re. antenna length; the length of
    the Rx antenna is calculated as a fraction of the transmitted
    wavelength and any alteration in this length can cause serious
    detuning.  That's why virtually all RC system operators' manuals warn
    us _NOT_ to double the antenna over its own length as this effectively
    shortens it and detunes the receiver.
    
    Purchase a length of stranded wire of the same AWG (gauge), cut it to the 
    exact length of the original antenna and replace the old/broken wire.  As 
    Jim so wisely suggests, put the Rx in a familiar (to the Rx) plane, go to a
    familiar place and range-check the system.  If you get the same range
    results as you previously got, go fly.  If not, send the system back to
    the mfgr. or an authorized service center for repair/retuning.
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      | |        00	       <|  ~~~  ____ 04 ---- | --------------------
    |_|_|        (O>o		|\)____/___|\_____|_/	   Adios amigos, Al
      |     \__(O_\_	        |	  |___/	 o	   (The Desert Rat)

    I kind of figured you say that!  Oh well, I can buy some wire of the
    same awg and solder it in place.
    
    One concern on doubling the antenna over itself....  I been flying like
    that for the last couple of years.  What I do is tie a slip knot in the
    wire and hook it to my vertical stab.  Then I slide the knot back down
    the wire to tighten it up.  This way here I don't have wire hanging all 
    over the place.  ....and now you tell me that's a no-no!  ...but in all
    this time I've never had a problem, so what gives?
    
    Ken

    Ken,
    
    All I can say is that you've been luck, seen-yore.  But, think back,
    have you ever had any [apparent] momentary losses of control you
    couldn't explain...occaional out-of-the-blue glitches, etc.?  These
    would definitely be the receiver telling you it was struggling to
    operate with a detuned antenna circuit.  If you truly never noticed a
    problem, you have indeed been fortunate!
    
    Never, NEVER double up the receiver antenna!!  If you have excess wire
    after stringing it to the vertical stab, either let it blow in the
    breeze or string it out to the tip of the stab.  This is far, FAR
    better than doubling the antenna over itself.  When you double the
    antenna, you'd just as soon cut the excess wire off and throw it away
    as that's exactly the effect you obtain by folding it double.  
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				|      |        / |\	   	       
      	         \|/		|______|__(o/--/  | \	   	       
      | |        00	       <|  ~~~  ____ 04 ---- | --------------------
    |_|_|        (O>o		|\)____/___|\_____|_/	   Adios amigos, Al
      |     \__(O_\_	        |	  |___/	 o	   (The Desert Rat)

    Ken,
    
    just to second the one-voice advice given so far - if you stretch the
    antenna to the tail feathers, it will never have the correct length. So
    if you tie a rubber band to the antenna, make sure the knot is in the
    rubber and _NEVER EVER_ in the antenna.
    
    This to avoid Holm- und Rippenbruch
    
    Hartmut

    Al,
    	What if I run the wire out to the vertical stab, up and around the 
    stab and back down to the base of the fuse...just to keep it from
    dangling all over the place.  In other words, it doesn't have to be in 
    a straight line....does it?
    
    Ken

    Harmut,
    	Thanks for the advice, but on my Super Chipmunk I have the antenna
    running through the fuse and out the back end.  I'm left with about
    3/4" of wire and I made a knot in the very end so it wouldn't work it's
    way back into the fuse.  I've got over 20 flights on the MUNK without
    incident.  Am I still safe?
    
    Ken
    
    PS.  Boy, you guys are making me wonder about my installation process.

    The whole thing with the antenna is not dual - work or don't work - it
    is ANALOG. If you cut the antenna 1/10th of it's whole length you might
    loose (just a coarse guess) 10% of your receiving energy. If you put a
    knot into the end of your antenna, the inductance of your antenna
    increases somewhat, thus your input LC cirquit and its resulting
    frequency is not 100% on the middle of your TX frequency, again its
    only percentagewise and probably hard to measure anyway (I never tried)
    
    A not 100% correct length of your antenna you might notice a difference
    when you get to the maximum distance between TX and RX. Again, in one
    case you might loose some percentage of this maximum distance. If this
    limit would be 2000 meters in the other case it would be reduced to 
    1800 meters in the air (what is too far to see your plane anyway)
    
    The same is with your TX antenna, if 0.2 meters out you might have 100
    meter, if 1.2 meters out you might get 600 meters (on the ground)
    
    I know, I know, it's not that simple and also not a linear function,
    but easier to understand.
    
    
    Bernd
    
    (the end ot my antenna in the KNIFE-edge, a pattern plane, looks like
     that                                          ____________
         ____________________________             /
                             ________)-----------<   elevator
                                  /\              \____________
                                   doubled with shrinkage tube and attached
                                   with rubber to the elevator - no
                                   Problems 
    
    whoes'zat ?
    
    
    
    

    Boy, I guess my brother has been extremely lucky...not only has he been
    flying with his antenna wire doubled, but he is also flying with about
    6" of his TX antenna missing!  I guess I better inform him.
    
    Ken

My experience with tuning the antenna circuit bears this
out...you can get a fairly coarse peak if the radio TX is near
the RX antenna, but if you remove the antenna and put the TX
further away, the peak is very, very, sharp.  The point of this
being that the antenna/tuning coil circuit is very closely
matched and changing the antenna characteristics will definitely
detune the circuit.

On the PT, I put a toothpick at the top of the vertical stab.
Then I slipped a piece of fuel tubing over the antenna and used
it like a rubber band to hold the antenna wire to the toothpick.
Works just fine.  I've also slipped a button over the antenna and
hooked a rubber band from it to an attachment point on the
vertical stab.  

    Well, I've been lookin' around for some 24awg stranded "copper" wire
    and have come up short.
    
    Question;  Does the wire have to be stranded vs. solid and does it
    matter if its not quite 24awg?  The length will be the same as the
    original.
    
    Ken (who knows nothing about antenna reception/tuning)

The wire should be stranded because solid wire does not take the
constant bending and vibration that an antenna, and actually all
rc wiring, takes.  I can't think of a single safe application for
solid wire in an RC model.

On the gauge; changing the diameter of the wire will change its
inductance, and therefore the tuning characteristics of the
circuit.  But you could probably go one size either way without
much harm -- that is, 22 to 26 gauge.

    RE: .-2, KEN,
    
    As John cautions, DO NOT even consider using solid/single-conductor
    wire...if nothing else, it'll break under the constant flexing and
    vibration a receiver antenna must endure.  As to the stranded wire, I
    can't imagine that you can't find stranded copper wire...I mean, what
    else do they use to make stranded electrical wire?  Any Radio Shack
    should be able to fill the bill.  Keep us posted.......
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				|      |        / |\	   	       
      	         \|/		|______|__(o/--/  | \	   	       
      | |        00	       <|  ~~~  ____ 04 ---- | --------------------
    |_|_|        (O>o		|\)____/___|\_____|_/	   Adios amigos, Al
      |     \__(O_\_	        |	  |___/	 o	   (The Desert Rat)

    John/Al, 
    	Thanks for the continued input.
    
    	I went to Radio Shack but they only sold a package of 3 rolls (red,
    green and blue).  I figured I could find something here at Digital
    since we do deal in electronic stuff. 
    
    	Well, I did find some wire which looks about the same size, but it
    was stiffer.  I examined the wire and found the strands to be thicker.
    The antenna wire has 20 strands of wire while the one I found has 7 
    strands of sliver wire.
    
    	Is this wire NO GOOD?
    
    Ken

    Re: .-1, Ken,
    
    Let's put it this way..._I_ sure wouldn't use it!  Try some of the
    other local electronics supply houses...what you want is 24 AWG
    "hook-up wire."  That stuff should be common as dirt.
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				|      |        / |\	   	       
      	         \|/		|______|__(o/--/  | \	   	       
      | |        00	       <|  ~~~  ____ 04 ---- | --------------------
    |_|_|        (O>o		|\)____/___|\_____|_/	   Adios amigos, Al
      |     \__(O_\_	        |	  |___/	 o	   (The Desert Rat)

    The advice given here has been good. You shouldn't double over the
    antenna. As far as following contours, it doesn't have to be in a 
    straight line, but I would recommending keeping any angles to
    greater than 90 degrees. Also avoid placing it near and parallel
    to any long metal objects such as pushrods, which could affect the
    antenna's function from different directions.
    
    Interestingly enough, when I sent my radio back to Airtronics, I
    asked for a new receiver antenna because the original had broken
    and been resoldered. They replaced it (no extra charge above the
    overhaul fee), and it wasn't exactly the same length. I run on
    53MHz, and the antenna isn't very different in length from
    the 72MHz ones. If the antennas were 1/4 wave long, that would be
    about 1 meter for 72MHz, and 1.5 meter for 53MHz. They are instead
    less than a meter long. So, I would guess that the antenna circuit
    is detuned for a "long wire" (ie, random length) rather than a 
    tuned-length antenna. The same goes for the transmitter antenna length.
    
    BUT, unless you're certain (like calling the manufacturer's engineering
    department) I would never change the length of the receiver antenna.
    
    Dave

Actually, you should be able to get it at the hobby shop.  ACE
sells a replacement antenna wire, intended for the Silver
Seven,but its the standard 24g 36" length, black wire.

My son has been having problems with his RC car.  He believes 
that it has limited range or is picking up other transmitters 
due to a sub-optimal receiving antenna length.  Of course, the 
hobby shop said his EE-father is to blame and hobby shops know 
best about antenna design.  Actually, I have run the car down 
the street for 100+ yards without loosing control, but it got 
confused at the track this weekend.

The car runs at 75 MHz and a 1/4-lamda antenna would be 38 to 40 
inches long.  The practical, physical length of the antenna
seems to be in the 14 to 18 inch region.  Solutions and 
practices suggested are as follows:  (I consider a thru d to be 
pure BS.)

a.  Cut the excess wire off, something that has been published 
    as and believed to be an absolute no-no.  (Naturally, this 
    is what EE-dad did, saying that is no worse than anything 
    else.)

b.  Wind the excess wire up at the base of the antenna (between 
    the antenna and receiver), which probably is no better.

c.  Fold the antenna over, with the excess taped to the outside 
    of the plastic antenna tube, also probably no better, but 
    a recommended and common practice.

d.  Buy a plastic loom on which to wind the excess wire, between 
    the antenna and the receiver.  This is the recommended "by 
    this" approach.  Is the loom designed to be a loading coil?  
    I doubt it.

e.  Wind the excess wire in a zig-zag, non-inductive fashion on 
    a cardboard loom.

f.  Purchase a base-loaded, 8-1/2 inch antenna, which is too 
    short, in most cases.

I'm wondering if someone can tell me how to make a continuously-
loaded antenna in the 75 MHz, 14-18 inch range or calculate the 
inductance needed for a base-loaded antenna.  

Has anyone worked with a loop-antenna, mounted around the inside
of the car body?

Has anyone had experience with interference from other, cheap 
transmitters splattering the band?  This may be another cause of 
his problems.

thanks, doug