antenna orientation question

Thanks everyone for the responses, which reinforce what I have found in
the field. Here's another one, tho: as we're all familiar with, the
whip antennas on small transmitters tend to bend over time, with some
of the more flexible ones developing kinks and "S" curves. How does a
change like this affect the antenna's tx/rx ability, if the whip stays
the same length but is a different shape?

And another: sometimes I carry an older Comtek transmitter (M-72) in my
over-the-shoulder bag. The long, semi-rigid VHF antenna (w/ BNC
connector) is awkward. Any suggestions for building a soft alternative?
I like the idea of a soft cable with BNC at one end and a clip at the
other to connect to the shoulder-harness strap. A coil-cable would be
ideal. However, I'm not versed enough in the issues at hand: wire
type/gauge, loss from coiled vs. straight, etc. Could coax work by
using the braid as the antenna?
Anyone feel like teaching?
ben
 >> Stay informed about: antenna orientation question 

ben wrote:

> Any RF-savvy folks know the effect of a Tx antenna oriented vertically
> downward (e.g., a beltpack upside-down) with the Rx antenna vertical as
> usual? I know that Tx at right-angle to Rx is bad news.
> Thanks,
> Ben Lowry
> Austin, TX
>
No problems. But don't sit on the antenna, and be careful it doesn't fall from
however you're mounitng it.

John
 >> Stay informed about: antenna orientation question 

"ben" <blowry.TakeThisOut@bga.com> wrote in message
news:1109651069.352439.185870@g14g2000cwa.googlegroups.com...
> Thanks everyone for the responses, which reinforce what I have found in
> the field. Here's another one, tho: as we're all familiar with, the
> whip antennas on small transmitters tend to bend over time, with some
> of the more flexible ones developing kinks and "S" curves. How does a
> change like this affect the antenna's tx/rx ability, if the whip stays
> the same length but is a different shape?
>
> And another: sometimes I carry an older Comtek transmitter (M-72) in my
> over-the-shoulder bag. The long, semi-rigid VHF antenna (w/ BNC
> connector) is awkward. Any suggestions for building a soft alternative?
> I like the idea of a soft cable with BNC at one end and a clip at the
> other to connect to the shoulder-harness strap. A coil-cable would be
> ideal. However, I'm not versed enough in the issues at hand: wire
> type/gauge, loss from coiled vs. straight, etc. Could coax work by
> using the braid as the antenna?
> Anyone feel like teaching?
> ben
>

Although I was making light of it, the actual best place for a Transmitter
antenna is above your head.
Years ago I was working on a Doc following around Coach Darryl Royal (you
probably don't know who that is)
around UT memorial stadium. I was carrying a Nagra 4.2 over the shoulder, a
Senn 815 in Rycote WS in one hand and 2 Vega wireless mic receivers.( The
big VHF kind that ran off C-Cells and weighed about 8 pounds each). The
Vega's were in the back pouch/pocket of a hunter's vest and I ran antenna
coax cables up to whip antennas I mounted on my headphones. They stuck
straight up above my head. I grounded the shields of the coax to the metal
band of the headphones forming the ground plane. Reception was great all
over the stadium.. This was before diversity and UHF wireless mics. I also
was allowed to go anywhere I wanted because I looked like I was from another
planet and no one wanted to challenge someone in such a bizarre rig.

Best advice is keep the coax antenna cables short (or none at all) and get
the antenna as far away from you body as you can. Above the head allows 360
degrees of transmission or reception and is not blocked by your body. Plus
if you are tall, it is above every other blocking body wandering around.
Coiled cables are bad. They change in length and the changing loops in coil
causes self cancellation and variable impedance. Straight whip is the most
efficient, cut to 1/4 or 1/2 a wavelength of the center frequency you are
transmitting. Example a simple 1/2 wave vertical can be calculated by 468/f
where f is the frequency in Megahertz. The answer is in feet A 1/4 wave
vertical would be 234/f etc. Of course the impedance of the feed cable
should be taken into account and may require base-loading coil to match the
impedance of your antenna.

--Courtney
 >> Stay informed about: antenna orientation question 

Interestingly enough i think some of the very first Vega units were a
helmet mounted TX antenna worn on the head of a model if i remember the
print ad correctly.
I cant imagine these being to popular though.

Mike Curtis
 >> Stay informed about: antenna orientation question 

"ben" <blowry DeleteThis @bga.com> wrote in message
news:1109637476.919241.296130@o13g2000cwo.googlegroups.com...
> Any RF-savvy folks know the effect of a Tx antenna oriented vertically
> downward (e.g., a beltpack upside-down) with the Rx antenna vertical as
> usual? I know that Tx at right-angle to Rx is bad news.
> Thanks,
> Ben Lowry
> Austin, TX
>

I believe it is better to have the ground-plane below the antenna. In most
transmitters the ground-plane is formed by the case of the transmitter and
once it gets soaked in sweat, the actor's body acts as such. I have found
Vertical to be best, and separated from the body as far as is feasible.
Usually not easily accomplished... Poking straight up from the top of the
head is a great position, just kinda hard to wear a Hat over it ;-)...

The Gurus from Lectro should chime in here for a more authoritative answer.

---Courtney
..
 >> Stay informed about: antenna orientation question 

Only thing I'd add is a specific statement that when indoors, antenna
orientation is not as critical as when the situation is line of sight
outdoors. Indoors, the RF signal is bounced around so much that
orientation is not as critical. In all cases though, you are safest
using vertical transmitter antennas with vertical receiver antennas.

As far as the question of whether the antenna should be above the
transmitter or below it, the only rule I'd follow is the higher the
antenna the better. Higher, upside down is better than lower, right
side up. Usually , right side up does get the antenna a little higher.

Larry Fisher
Lectrosonics

On 28 Feb 2005 19:59:15 -0800, opusstuf.DeleteThis@cableone.net wrote:

>Hi Ben:
>
>First, thanks to John and Courtney for useful replies.
>
>The RF from a transmitter antenna radiates perpendicular to the axis of
>the antenna, regardless of whether the whip (aerial) is vertical or
>horizontal. As long as both receiver and transmitter antennas are
>vertical, they should work fine. With respect to a belt-pack Tx, it
>works much like a dipole antenna, with the whip being one half and the
>housing of the transmitter being the other half. As Courtney
>mentioned, when the housing of the transmitter is in close contact with
>the user's body, it starts behaving like a ground plane configuration
>(assuming the whip is not in contact with the user's body).
>
>In actual use, the RF signal bounces off surfaces anywhere in the
>vicinity (multipath reflections) and the signal that finally arrives at
>the receiver antenna may come from every direction. As the signals
>arrive at the receiver, they add and subtract from each other (phase
>cancellations and additions). This scenario is what a diversity
>receiving system handles. Antenna <polarization> in common use is only
>one of many factors that affect the signal strength that arrives at the
>receiver antenna. Courtney's suggestion of keeping the antenna away
>from the user's body will often have a greater effect on signal
>strength than the other factors.
>
>Outdoors, away from nearby metallic surfaces, and with no obstructions
>between the Tx antenna and the Rx antenna, polarization does come into
>play. The height above the ground also affects the radiated RF power
>from the Tx antenna.
>
>The bottom line is simply to monitor the RF signal strength at the
>receiver. Make adjustments as best you can to get the highest RF level
>at the receiver. Modern receivers are very sensitive and can deliver
>usable audio at even a few microvolts.
>
>Larry may chime in here and give you some more useful technical advice.
>
>Best regards,
>Bruce
 >> Stay informed about: antenna orientation question 

Hi Ben,
As long as the bends are moderate, the bends don't affect the antenna
much at all. The overall length and distance to the ground plane
establishes the capacitance of the antenna and the length establishes
the inductance. Neither of these vary much with small bends in the
wire. The capacitance and inductance establish the resonate frequency
of the antenna which is the most critical factor. If the wire gets very
close to metal (or the ground plane) then the capacitance to ground
increases greatly. If the wire makes a loop or folds back on itself
then the inductance will also increase a lot. Either of these would
lower the resonance below the "cut" frequency and the antenna would not
work as well.

The cable in our antennas can be dekinked by bending it with your
fingers and you can do this hundreds if not thousands of times without
breaking the wire.
This is FAQ #56 on our site now. Good question.
Larry Fisher
Lectrosnics

ben wrote:
> Thanks everyone for the responses, which reinforce what I have found
in
> the field. Here's another one, tho: as we're all familiar with, the
> whip antennas on small transmitters tend to bend over time, with some
> of the more flexible ones developing kinks and "S" curves. How does a
> change like this affect the antenna's tx/rx ability, if the whip
stays
> the same length but is a different shape?
>
[snip]
> ben
 >> Stay informed about: antenna orientation question 

opusstuf.TakeThisOut@cableone.net wrote:

> Hi Ben:
>
> First, thanks to John and Courtney for useful replies.
>
> The RF from a transmitter antenna radiates perpendicular to the axis of
> the antenna, regardless of whether the whip (aerial) is vertical or
> horizontal.

It gets just a little more complicated than that, too. Since radio is an
electromagnetic wave, it is really two waves, the E field and the H field. When
talking about antenna polarization, we're always talking about the E
[electrical] field.

As long as both receiver and transmitter antennas are
> vertical, they should work fine. With respect to a belt-pack Tx, it
> works much like a dipole antenna, with the whip being one half and the
> housing of the transmitter being the other half. As Courtney
> mentioned, when the housing of the transmitter is in close contact with
> the user's body, it starts behaving like a ground plane configuration
> (assuming the whip is not in contact with the user's body).
>
> In actual use, the RF signal bounces off surfaces anywhere in the
> vicinity (multipath reflections) and the signal that finally arrives at
> the receiver antenna may come from every direction.

And when a radio signal reflects off something, it goes through a 90 degree
phase change.

As the signals
> arrive at the receiver, they add and subtract from each other (phase
> cancellations and additions). This scenario is what a diversity
> receiving system handles. Antenna <polarization> in common use is only
> one of many factors that affect the signal strength that arrives at the
> receiver antenna. Courtney's suggestion of keeping the antenna away
> from the user's body will often have a greater effect on signal
> strength than the other factors.

And a sweaty actor with a damp transmitter absorbs more RF than a dry one!
>
> Outdoors, away from nearby metallic surfaces, and with no obstructions
> between the Tx antenna and the Rx antenna, polarization does come into
> play. The height above the ground also affects the radiated RF power
> from the Tx antenna.

When you run out of "rope" with radios, the easiest and cheapest way to get more
range is to raise the receiver a couple of meters, and run audio cable to the
mixer. This is preferable to just raising the antenna, unless you have very good
coax [RG 58 attenuates a 700MHz signal about 17dB per hundred feet] and an
antenna made for mounting remotely. A simple rubber duckie will need a ground
plane or other counterpoise if remoted. And everybody has another 20' piece of
XLR cable in their kit ;-p Most people with quad boxes have dipole or LPDA
antennas that feed all the recievers, and they're made to be mounted up on a
boom with coax feeds.
>
> The bottom line is simply to monitor the RF signal strength at the
> receiver. Make adjustments as best you can to get the highest RF level
> at the receiver. Modern receivers are very sensitive and can deliver
> usable audio at even a few microvolts.
>
> Larry may chime in here and give you some more useful technical advice.
>
> Best regards,
> Bruce
>
What he said.

John
 >> Stay informed about: antenna orientation question 

"Mike Curtis" <mike.DeleteThis@prosound.tv> wrote in message
news:1109661133.290023.33810@o13g2000cwo.googlegroups.com...
> Interestingly enough i think some of the very first Vega units were a
> helmet mounted TX antenna worn on the head of a model if i remember the
> print ad correctly.
> I cant imagine these being to popular though.
>
> Mike Curtis
>

These were used quite extensively in News coverage in the early days of
wireless mics.
Remember the headsets Dan Rather wore at the Conventions when he was ejected
back in the 60's. They were used in sports stadiums for roving reporters
around the field, at the olympics and many other network venues. Yes they
did look pretty much like some Buck Rogers communication Device.
---Courtney
 >> Stay informed about: antenna orientation question 

Courtney,
What's the "gutted coax" idea you mention? I'm imagining, maybe, using
the center conductor and insulator, with the braid/shield and outer
insulation stripped away?
ben
 >> Stay informed about: antenna orientation question 

"ben" <blowry.DeleteThis@bga.com> wrote in message
news:1109741649.438697.316350@o13g2000cwo.googlegroups.com...
> Courtney,
> What's the "gutted coax" idea you mention? I'm imagining, maybe, using
> the center conductor and insulator, with the braid/shield and outer
> insulation stripped away?
> ben
>

Larry wrote a very good description of how to create a cheap flexible
antenna in another thread titled oddly enough: "Ben's Question about a Soft
antenna"
In my mail reader it follows this thread by a few..

----Courtney
 >> Stay informed about: antenna orientation question 

d'oh!
thanks,
ben
 >> Stay informed about: antenna orientation question 

"Larry Fisher" <larryf.DeleteThis@despammed.com> wrote in message
news:trn921pedqo3vmt3i0ft0abt4een9nbc9a@4ax.com...
> Hi Courtney,
> Good post about the alien antenna rig. I can just imagine. I would pay
> to see a picture of that setup.
>
> Your prohibition against coiling cables was probably for very tight
> coils. However, I know some users have been concerned about the
> inductance or loss of "reasonable" sized coil loops. There have also
> been concerns about laying coax on metal or running it by a piece of
> metal.


Larry,
My prohibition against coiled Cables was not about coiling a Coax feed
cable, but I was addressing Ben's problem of needing a soft antenna and his
using a "Coiled cable" (like a telephone handset wire) as the radiating
portion of an antenna for his Comtek. This is the kind of Coiled cable I
was saying would be bad. With a Coiled Radiator (unshielded) the antenna
would loop back onto itself to varying degrees as the coil stretched and
relaxed it would vary the impedance and VSWR of the antenna causing large
variance in efficiency of the transmitter.

If Ben wants a soft cable he could use your "gutted Coax" idea and attach a
rubber band or 2 to the end of the center conductor and attach the other end
of the rubber band to the shoulder strap of his bag setup to keep the
flexible center conductor relatively straight and vertical and the rubber
band prevents the tied down cable from breaking when stretched tight.
----Courtney
 >> Stay informed about: antenna orientation question