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Adapting "Ext." antenna to wireless mike RX's with attached only antennae

T

TedL

Hi, Engineers...

Am looking for suggestions/tips about adding external antennas to several wireless mike receivers that have no provision for such... just box-top "rabbit ear" style elements.

I'm trying to overcome frequent signal dropouts due to the receivers being located (by someone else) in a metal swingaway wall rack that does a pretty good job of shielding. (Sigh.)

The companion mikes are usually used at the other end of the next room... which doesn't make for good signal strength at the receivers and causes frequent dropouts.

Two are Shure T4A-CC diversity receivers with dual "rabbit ear" elements on each side of the receiver box.
http://www.shure.com/wireless/pg/t4.asp

One is on 177.600, the other on 186.200.

The third unit is an Audio Technica ATW 601B/L package operating on one of ten user selectable channels in the 482-492MHz band... TV ch. 16-17.

http://www.audio-technica.com/cms/wls_systems/b9f90e96dc8108cf/index.html

This one is not a true diversity system, but has two elements on opposite sides of the receiver case.

One obvious idea is remove the "rabbit ears" and attach 50 or 75 ohm coax to the rabbit ear connection, and tying the shield to the ground of the PC board.

Or... put BNC or UHF jacks on the top of the reciever and connect as above.

I'm thinking I'd terminate the other end in a UHF or BNC connector, to which I'd add a mating connector with a length of copper or stainless steel cut to match the wavelentgh (or fraction) of the operating freq. for that receiver.

I plan to mount the array about 15-20 feet above the floor on the back wall of the room where the use takes place, giving good line of sight and elevation.

Elements would probably be vertical, since that would be easiest to mount.

-----???
Suggestions on antenna element length for the three frequencies, and on spacing the external elements for the diversity units?
-----???

An electric basketball scoreboard you'd see in school gyms is near a hole in the wall I'd make use of for feeding the coax through.

Thoughts about whether to use the top of the scoreboard as a "shelf" from which I can mount the array would be appreciated. I don't want to have the scoreboard become a source of interference or ground loops.

I can use some form of insulating material like plexiglass to make the attachment, positioned to avoid damage from stray basktetballs, or just find the wall studs and mount brackets through the drywall to the studs and then make a narrow shelf.

I wote that thinking I'd use chassis mount connectors for the "bottom" end.

Now that I re-read that, I could make it even simpler:

Use nylon cable clamps to screw the connectors and the tops of the elements into the drywall or studs or a furring strip. That would keep them up against the wall with a minimum of attachment complexity and RF issues.

Thanks in advance for your help. I'm looking forward to your suggestions.

Ted
 
> Hi, Engineers...
>
> Am looking for suggestions/tips about adding external
> antennas to several wireless mike receivers that have no
> provision for such... just box-top "rabbit ear" style
> elements.
>
> I'm trying to overcome frequent signal dropouts due to the
> receivers being located (by someone else) in a metal
> swingaway wall rack that does a pretty good job of
> shielding. (Sigh.)
>
> The companion mikes are usually used at the other end of the
> next room... which doesn't make for good signal strength at
> the receivers and causes frequent dropouts.
>
> Two are Shure T4A-CC diversity receivers with dual "rabbit
> ear" elements on each side of the receiver box.
> http://www.shure.com/wireless/pg/t4.asp
>
> One is on 177.600, the other on 186.200.
>
> The third unit is an Audio Technica ATW 601B/L package
> operating on one of ten user selectable channels in the
> 482-492MHz band... TV ch. 16-17.
>
http://www.au> dio-technica.com/cms/wls_systems/b9f90e96dc8108cf/index.html
>
>
> This one is not a true diversity system, but has two
> elements on opposite sides of the receiver case.
>
> One obvious idea is remove the "rabbit ears" and attach 50
> or 75 ohm coax to the rabbit ear connection, and tying the
> shield to the ground of the PC board.
>
> Or... put BNC or UHF jacks on the top of the reciever and
> connect as above.
>
> I'm thinking I'd terminate the other end in a UHF or BNC
> connector, to which I'd add a mating connector with a length
> of copper or stainless steel cut to match the wavelentgh (or
> fraction) of the operating freq. for that receiver.
>
> I plan to mount the array about 15-20 feet above the floor
> on the back wall of the room where the use takes place,
> giving good line of sight and elevation.
>
> Elements would probably be vertical, since that would be
> easiest to mount.
>
> -----???
> Suggestions on antenna element length for the three
> frequencies, and on spacing the external elements for the
> diversity units?
> -----???
>
> An electric basketball scoreboard you'd see in school gyms
> is near a hole in the wall I'd make use of for feeding the
> coax through.
>
> Thoughts about whether to use the top of the scoreboard as a
> "shelf" from which I can mount the array would be
> appreciated. I don't want to have the scoreboard become a
> source of interference or ground loops.
>
> I can use some form of insulating material like plexiglass
> to make the attachment, positioned to avoid damage from
> stray basktetballs, or just find the wall studs and mount
> brackets through the drywall to the studs and then make a
> narrow shelf.
>
> I wote that thinking I'd use chassis mount connectors for
> the "bottom" end.
>
> Now that I re-read that, I could make it even simpler:
>
> Use nylon cable clamps to screw the connectors and the tops
> of the elements into the drywall or studs or a furring
> strip. That would keep them up against the wall with a
> minimum of attachment complexity and RF issues.
>
> Thanks in advance for your help. I'm looking forward to
> your suggestions.
>
> Ted
>

Here's the trick of the day. There's no easy way of knowing what the actual impedence is at the point you are picking the antenna signal off to run to the coax connector. This makes it tricky as you could end up with many things that are NOT anywhere close to 50 ohms. I've ran into this problem with many wireless things that had fixed antennas that I've "converted". Sometimes it works O.K. and sometimes it doesn't. There's no easy way to know really until you try I guess. Most of the time anything would be better though than keeping it within a sheilded enviorment with the afixed antennas.

Good luck!
 
> > Hi, Engineers...
> >
> > Am looking for suggestions/tips about adding external
> > antennas to several wireless mike receivers that have no
> > provision for such... just box-top "rabbit ear" style
> > elements.
> >
> > I'm trying to overcome frequent signal dropouts due to the
>
> > receivers being located (by someone else) in a metal
> > swingaway wall rack that does a pretty good job of
> > shielding. (Sigh.)
> >
> > The companion mikes are usually used at the other end of
> the
> > next room... which doesn't make for good signal strength
> at
> > the receivers and causes frequent dropouts.
> >
> > Two are Shure T4A-CC diversity receivers with dual "rabbit
>
> > ear" elements on each side of the receiver box.
> > http://www.shure.com/wireless/pg/t4.asp
> >
> > One is on 177.600, the other on 186.200.
> >
> > The third unit is an Audio Technica ATW 601B/L package
> > operating on one of ten user selectable channels in the
> > 482-492MHz band... TV ch. 16-17.
> >
> http://www.au>
> dio-technica.com/cms/wls_systems/b9f90e96dc8108cf/index.html
>
> >
> >
> > This one is not a true diversity system, but has two
> > elements on opposite sides of the receiver case.
> >
> > One obvious idea is remove the "rabbit ears" and attach 50
>
> > or 75 ohm coax to the rabbit ear connection, and tying the
>
> > shield to the ground of the PC board.
> >
> > Or... put BNC or UHF jacks on the top of the reciever and
> > connect as above.
> >
> > I'm thinking I'd terminate the other end in a UHF or BNC
> > connector, to which I'd add a mating connector with a
> length
> > of copper or stainless steel cut to match the wavelentgh
> (or
> > fraction) of the operating freq. for that receiver.
> >
> > I plan to mount the array about 15-20 feet above the floor
>
> > on the back wall of the room where the use takes place,
> > giving good line of sight and elevation.
> >
> > Elements would probably be vertical, since that would be
> > easiest to mount.
> >
> > -----???
> > Suggestions on antenna element length for the three
> > frequencies, and on spacing the external elements for the
> > diversity units?
> > -----???
> >
> > An electric basketball scoreboard you'd see in school gyms
>
> > is near a hole in the wall I'd make use of for feeding the
>
> > coax through.
> >
> > Thoughts about whether to use the top of the scoreboard as
> a
> > "shelf" from which I can mount the array would be
> > appreciated. I don't want to have the scoreboard become a
> > source of interference or ground loops.
> >
> > I can use some form of insulating material like plexiglass
>
> > to make the attachment, positioned to avoid damage from
> > stray basktetballs, or just find the wall studs and mount
> > brackets through the drywall to the studs and then make a
> > narrow shelf.
> >
> > I wote that thinking I'd use chassis mount connectors for
> > the "bottom" end.
> >
> > Now that I re-read that, I could make it even simpler:
> >
> > Use nylon cable clamps to screw the connectors and the
> tops
> > of the elements into the drywall or studs or a furring
> > strip. That would keep them up against the wall with a
> > minimum of attachment complexity and RF issues.
> >
> > Thanks in advance for your help. I'm looking forward to
> > your suggestions.
> >
> > Ted
> >
>
> Here's the trick of the day. There's no easy way of knowing
> what the actual impedence is at the point you are picking
> the antenna signal off to run to the coax connector. This
> makes it tricky as you could end up with many things that
> are NOT anywhere close to 50 ohms. I've ran into this
> problem with many wireless things that had fixed antennas
> that I've "converted". Sometimes it works O.K. and
> sometimes it doesn't. There's no easy way to know really
> until you try I guess. Most of the time anything would be
> better though than keeping it within a sheilded enviorment
> with the afixed antennas.
>
> Good luck!
>
There are 2 other potential options also. The better option, although not cheaper, is to replace the receivers with receivers that offer external ant. ports. You can then use a combiner to attach multiple receivers to one antenna set that can be located in a more suitable location.

The next option is to relocate the current receivers to the top of the scoreboard and simply run the audio lines back to the rack. You can run power from the rack distro to control power to the receivers. I would be sure that the receivers are powered on the same circuit as the rest of the equipment to prevent ground noise (hum and buzz)

Using the method of replacing the built in antennas, remember to factor in feedline losses. depending on the length of feedline, you may have less signal reaching the receriver than you do now. also remember that antennas have 2 parts, the actual "whip" and the ground plane. The shield of the coax attaches to the groundplane, the center to the whip. If you just tie the shield to the chassis ground, you will end up with a very mismatched antenna. The longer the coax run, the more critical this may become. But as noted by the previous poster, you may or may not be dealing with a true 50 ohm load anyway.

as far as the length of the whip and whip material, the easiest is th simply strip the outer sheath of the coax to 1/4 wavelength of the desired freq. fold back the braided part and leave the center conductor alone. the conductor itself becomes the "whip". split the braid in half and extend it out either side of the center so it forms an upside down T. staple this to the wall.Congrats, you have made a simple quarterwave groundplane antenna! On the receiver end, tie the center to the whip, this the shield to wherever the shield is tied inside the unit (it may be a copper patch on the PCB)

Quarterwave antenna should be:
16.62 inches for 177.6 mhz
15.85 inches for 186.2 mhz
6.06 inches for 487 mz (center of the 482-492 range)

I would make a pair of 16 inch antennas and split them to both vhf receivers, and a pair of 6 in antennas for the uhf unit. I would use the best 50 ohm cable i could and keep the run as short as possible. At least RG-8 or 213.

Hope this helps,

-Parsons
 
Thanks for the info, both of you.

I'll let you know what happens when I get a chance to pursue. Busy with some video projects.

Cost prohibits replacing the TX/RX packages, although that's what I'd have preferred. Too bad the original install wasn't spec'd for units with external antennas.

Unfortunately, putting the receivers above the scoreboard isn't practical due to the active use of the gym/hall for basketball, (SMACK! "oops.") and the potential for theft.

I had considered simply stripping the coax as you describe (thnkx for the measurements)and may do that as a test to see what happens, then dress things a little more.

BTW: The Audio-Technica unit uses the two elements as an active element and a ground plane.

Ted.


> >
> There are 2 other potential options also. The better option,
> although not cheaper, is to replace the receivers with
> receivers that offer external ant. ports. You can then use a
> combiner to attach multiple receivers to one antenna set
> that can be located in a more suitable location.
>
> The next option is to relocate the current receivers to the
> top of the scoreboard and simply run the audio lines back to
> the rack. You can run power from the rack distro to control
> power to the receivers. I would be sure that the receivers
> are powered on the same circuit as the rest of the equipment
> to prevent ground noise (hum and buzz)
>
> Using the method of replacing the built in antennas,
> remember to factor in feedline losses. depending on the
> length of feedline, you may have less signal reaching the
> receriver than you do now. also remember that antennas have
> 2 parts, the actual "whip" and the ground plane. The shield
> of the coax attaches to the groundplane, the center to the
> whip. If you just tie the shield to the chassis ground, you
> will end up with a very mismatched antenna. The longer the
> coax run, the more critical this may become. But as noted by
> the previous poster, you may or may not be dealing with a
> true 50 ohm load anyway.
>
> as far as the length of the whip and whip material, the
> easiest is th simply strip the outer sheath of the coax to
> 1/4 wavelength of the desired freq. fold back the braided
> part and leave the center conductor alone. the conductor
> itself becomes the "whip". split the braid in half and
> extend it out either side of the center so it forms an
> upside down T. staple this to the wall.Congrats, you have
> made a simple quarterwave groundplane antenna! On the
> receiver end, tie the center to the whip, this the shield to
> wherever the shield is tied inside the unit (it may be a
> copper patch on the PCB)
>
> Quarterwave antenna should be:
> 16.62 inches for 177.6 mhz
> 15.85 inches for 186.2 mhz
> 6.06 inches for 487 mz (center of the 482-492 range)
>
> I would make a pair of 16 inch antennas and split them to
> both vhf receivers, and a pair of 6 in antennas for the uhf
> unit. I would use the best 50 ohm cable i could and keep the
> run as short as possible. At least RG-8 or 213.
>
> Hope this helps,
>
> -Parsons
>
 
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