Super-whippy whip antennas

[Paul Burridge]

Hi chaps,

Can anyone suggest a suitable material from which to make an
ultra-flexible mobile whip antenna say about 3 to 4 feet long. I need
something that can be bent to 90 degrees at a very small radius and
still return to reasonable straightness.

Thanks,

p.

 

[Active8]

I think thin piano wire will give the best results. Place a lot of them
in a flexible tube to gain some thickness and have them silvered for
conductivity - or add a stranded copper wire.

Spring metal strip may also work, especially if you can manage some type
of hinge at the base that allows it to turn when sideways force is
exerted, or if the force is in one plane.


Thomas

what is a tight bend? music wire comes in diff sizes and the number in
the tube and how tightly the tube conforms to it will have an effect on
the flexibility.

a shorty could be made from those old storm door return springs inside
shink tube which is what i'd suggest putting the music wire in. use the
copper wire, i'd think. seal the ends from the elements, except for fire
:-0

short segments of wire(s) with return spring segments as connectors?

OTOH, mobile whips with base springs are common enough if all you're
worried about is those friggin' trees hanging over the road that no one
cuts, or the drive through windows. why such a tight bend spec?

regards,
mike

 

[Paul Burridge]

what is a tight bend? music wire comes in diff sizes and the number in
the tube and how tightly the tube conforms to it will have an effect on
the flexibility.

Yeah, sorry chaps, I should have better defined what I meant by 'very
small radius'. I compete in radio-controlled model battles, so the
model stands to get fipped upside down from time to time in a very
bruising environment and it already has a very low ground-clearance,
so using a spring as a base mounting won't help much, I'm afraid.
We're talking about radiuses of as little as 5mm! How about the
specific grade of stainless steel in wire form that springs are made
from? I know where I can get hold of some of that....

 

[cpemma]

Yeah, sorry chaps, I should have better defined what I meant by 'very
small radius'. I compete in radio-controlled model battles, so the
model stands to get fipped upside down from time to time in a very
bruising environment and it already has a very low ground-clearance,
so using a spring as a base mounting won't help much, I'm afraid.
We're talking about radiuses of as little as 5mm! How about the
specific grade of stainless steel in wire form that springs are made
from? I know where I can get hold of some of that....
"I believe history will be kind to me, since I intend
to write it." - Winston Churchill

The alloy is Type 301 (17% chromium, 7% nickel stainless steel) hard drawn
wire, or even better is a 17/7PH (precipitation hardening) grade that our
firm once produced for tank aerials, that also may take a few knocks.

Both (especially the PH grade) need a final heat-treatment (420C for a few
minutes IIRC) for ultimate spring properties, but to get such a tight bend
you'd need quite a thin wire. It's a matter of the proof strain or limit of
proportionality, how much the outer skin can stretch without taking a
permanent set, compared to the neutral central axis, on the bend.

 

[Allodoxaphobia]

Yeah, sorry chaps, I should have better defined what I meant by 'very
small radius'. I compete in radio-controlled model battles, so the
model stands to get fipped upside down from time to time in a very
bruising environment and it already has a very low ground-clearance,
so using a spring as a base mounting won't help much, I'm afraid.
We're talking about radiuses of as little as 5mm! How about the
specific grade of stainless steel in wire form that springs are made
from? I know where I can get hold of some of that....

You might try a section from a steel tape measure.
(Just don't select one of the cheap, Made-In-China plastic/mylar
ones. HI!HI!)
Additionally, try to mount the antenna in a "well" on the robot.
Even a well of 5-10 mm will help out.

HTH,
Jonesy

 

[Active8]

Yeah, sorry chaps, I should have better defined what I meant by 'very
small radius'. I compete in radio-controlled model battles, so the
model stands to get fipped upside down from time to time in a very
bruising environment and it already has a very low ground-clearance,
so using a spring as a base mounting won't help much, I'm afraid.
We're talking about radiuses of as little as 5mm! How about the
specific grade of stainless steel in wire form that springs are made
from? I know where I can get hold of some of that....

oh, hi. it's the battle bot guy. i should have caught that, duh!

mike

 

[Paul Burridge]

The alloy is Type 301 (17% chromium, 7% nickel stainless steel) hard drawn
wire, or even better is a 17/7PH (precipitation hardening) grade that our
firm once produced for tank aerials, that also may take a few knocks.

Both (especially the PH grade) need a final heat-treatment (420C for a few
minutes IIRC) for ultimate spring properties, but to get such a tight bend
you'd need quite a thin wire. It's a matter of the proof strain or limit of
proportionality, how much the outer skin can stretch without taking a
permanent set, compared to the neutral central axis, on the bend.

There speaks a man who obviously knows what he's talking about. Yes, I
believe we can order 301 from our guy in N. London who's very
accommodating on such matters. Thanks for a valuable steer!
Just one point, though: is stainless steel a reasonable radiator of RF
energy?

 

[Paul Burridge]

You might try a section from a steel tape measure.
(Just don't select one of the cheap, Made-In-China plastic/mylar
ones. HI!HI!)
Additionally, try to mount the antenna in a "well" on the robot.
Even a well of 5-10 mm will help out.

Not sure about the tape measure suggestion, but mounting in a well is
something I'd not considered and am most grateful for the idea of!
Thanks...

 

[Ralph Mowery]

:

There speaks a man who obviously knows what he's talking about. Yes, I
believe we can order 301 from our guy in N. London who's very
accommodating on such matters. Thanks for a valuable steer!
Just one point, though: is stainless steel a reasonable radiator of RF
energy?

Back years ago (30 or so) there was an ad in a magazine about either a
loaded CB or 5/8 two meter whip about 36 inches long bent in a circle made
of the 17/7 material. Not sure what the whips are actually made of but
most are some kind of steel .

 

[john graesser]

Not sure about the tape measure suggestion, but mounting in a well is
something I'd not considered and am most grateful for the idea of!
Thanks...

Does it have to be a whip antenna? Why not try a horizontal circular loop?
Since you are working line of sight to the robot, the signal loss from going
from vertical to horizontal shouldn't matter much unless your transmitter is
extremely low power. A loop could be mounted inside a wooden or fiberglas
body and be pretty safe from attack.

Is there anything in the rules forbidding your installing a jammer
transmitter to cause the other bot to lose its command channel? Probobly a
poor use of what little electrical capacity the onboard battery holds, but
it would be one way of causing the other bot to freeze in place and become
like a deer in the headlights.
thanks, John.
KC5DWD

 

[Paul Burridge]

Does it have to be a whip antenna? Why not try a horizontal circular loop?
Since you are working line of sight to the robot, the signal loss from going
from vertical to horizontal shouldn't matter much unless your transmitter is
extremely low power. A loop could be mounted inside a wooden or fiberglas
body and be pretty safe from attack.

If I believed this idea was workable I'd have implemented it by now.
You need to bear in mind that immediately beneath the polycarbonate
surface armor, there's a hulking great metal framework. Consequently,
sandwiching the antenna between the armour and the frame is going to
lead to unacceptable loss of radiated energy, I'd have thought. Unless
anyone knows differently..

Is there anything in the rules forbidding your installing a jammer
transmitter to cause the other bot to lose its command channel? Probobly a
poor use of what little electrical capacity the onboard battery holds,

There's a considerable amount of battery power on board, actually,
since the peak current draw is well over 100 Amps at times. A few
milliwatts for a local jammer would therefore be a negligable drain on
resources. *However* as you've already guessed, jammers are banned, as
are EMP pulse type weapons and such like. The reason for this is very
sound, when you think about it: it makes for really bad TV. The
producers want to see as much *action* as possible. You ain't gonna
get that if everybody's disabled everybody else's robot!

 

[cpemma]

Stainless steel is awful at conducting electricity - so it won't make
a very efficient antenna. But how much it really matters, I don't
know.

So's my body, but I get a decent TV picture if I touch the aerial input ;-)

 

[Richard Henry]

i always thought jamming would be a great weapon, but i bet it's
illegal. i'd like to see the rules for some of those competitions. i'd
think it would be fun to try sometime. i wan't to smash jay leno's bot.
come on. "chinkilla"? who writes his lines?

Paul's restricted to two bands, which i think stinks. he's probably not
allowed to jam. The band restriction i really don't like since it's a
bot competition not a DX contest.

I have a friend who competes. I suggested jamming to him; he said it
was not allowed.

Besides, they're not really robots. All of them are just RC toys. I'll
be more impressed when they are autonomous.

 

[Paul Burridge]

I doubt that the difference in resistance between stainless and copper
is going to cause any significant additional losses in a whip antenna.
That difference will be small compared to the other losses. Stainless
is often used for whip antennas because of its mechanical properties.

Yes, I've several proprietory s/steel whip antennas that give
excellent results. But a question springs to mind. If a s/steel
antenna has slightly higher resistance, does that appreciably lower
its Q?

 

[Paul Burridge]

Besides, they're not really robots. All of them are just RC toys. I'll
be more impressed when they are autonomous.

That's not entirely true these days. Autonomy in certain areas of
control is becoming increasingly prevalent. It'll be interesting to
see how this particular aspect of design develops over the coming
years...

 

[Active8]

Yes, I've several proprietory s/steel whip antennas that give
excellent results. But a question springs to mind. If a s/steel
antenna has slightly higher resistance, does that appreciably lower
its Q?

you mean it's bandwidth? probably, but why worry? it's not cut to
resonance, anyway. it's a resistor in an E field. Tx units can be run
with the telescoping verticle collapsed and still control the toy at
short ranges. i think the biggest prob would be front end overload
regardless of the antenna, which i'd try to deal with in an active
antenna setup which is well suited to short verticles, you just can't
let it blast the front end of one of those little RC Rxs.

i've used all kinds of different scraps of wire to get FM RXs going to
see if they work. how about those little pieces of scrap wire on RC
cars? they're no better than yours.

BR,
mike

 

[Bob Lewis \(AA4PB\)]

If a s/steel antenna has slightly higher resistance, does that

appreciably lower its Q?

I don't have any figures available but I wouldn't think so. The
difference between the resistance of a 2-foot piece of copper and a
2-foot piece of stainless is going to be pretty small.

 

[Roy Lewallen]

The short answer is that stainless probably won't make a noticeable
difference in loss, and therefore won't make a significant difference in Q.

The conductor loss of an antenna of a given wavelength size gets less as
the frequency gets higher. That's because the antenna length decreases
in inverse proportion to the frequency, while the loss per unit length
increases only as the square root of the frequency. So for a given wire
diameter, a half wavelength dipole at, say, 10 MHz has half the loss of
a half wavelength dipole at 2.5 MHz. Stainless steel whips are fine at 2
meters. But an 80 meter dipole made from small or moderate gauge
stainless wire could be pretty lossy -- almost certainly so, if the
stainless is a magnetic alloy. (Being magnetic greatly increases the RF
loss -- by a factor of the square root of the permeability.)

Loss becomes very important when a whip is a lot shorter than a quarter
wavelength. However, in many or most cases (like an HF mobile whip), the
whip typically has a fairly large diameter where the current is high,
and the whip loss is swamped by other losses, so the whip loss isn't
objectionable.

It's always possible to come up with a combination of whip diameter,
length, and frequency where stainless could be a poor choice -- but it's
uncommon in typical applications.

Roy Lewallen, W7EL

 

[Wim Ton]

surface armor, there's a hulking great metal framework. Consequently,

sandwiching the antenna between the armour and the frame is going to
lead to unacceptable loss of radiated energy, I'd have thought. Unless
anyone knows differently..

I do not know the frequencies, but if they are high enough, one could think
of a slot or a patch antennaIf jamming would be allowed, it would be a totally different sport (but
interesting as well). In that case you may as well omit the robots See all
the military history about ECM, ECCM, ECCCM (Electronic Counter(*n)
Measures) etc.

Wim

 

[Wim Ton]

Stainless steel is awful at conducting electricity - so it won't make a

very efficient antenna. But how much it really matters, I don't know.

You can coat it with silver or copper, only the outer few microns conduct HF
(skin-effect)

Wim