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Oct 11, 2008, 11:51 AM
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Old Man Mike's Avatar
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Simple Solution to Range Video 900 Mhz Antenna Issue


Here is a simple way to improve the performance of the Range Video 900 Mhz Antenna and reduce
interference to GPS receivers. I’ll keep this first post simple and will follow it with a more in depth
discussion on how it works. This modification costs under $15 and will provide the following
improvements:

1) Retunes the antenna to provide a better match to the antenna. The transmitter will run cooler and
put out more power. (SWR improved from 3.5 down to less than 2.0).

2) Reduces interference to GPS receiver by more than 100 times.

3) Improves efficiency of Antenna by a better ground (counterpoise).

4) Reduces interference in the 2.4 Ghz band by a factor of 2 times.

All that is required is a standard SMA “T-connector” and a short piece of coax semi hardline (UT-141) cable
cut from surplus coax jumpers:



The SMA T-connector is available here for $5 and $1 shipping here:

http://cgi.ebay.com/T-connector-SMA-...3286.m20.l1116

The UT-141 coax is available from Down East Microwave. You have to call and ask for the surplus UT-141
coax jumpers. You can cut two of the sections required for this modification from one jumper so that will
give you a spare. Cost is $2 per jumper and $5 shipping. Their website with contact info:

http://www.downeastmicrowave.com/

Here is a picture showing the precise length of cut coax how everything connects:



Finally, the following is a short video showing how well the modification works to reduce interference to a
GPS receiver located 6 inches from the transmitter. You can see that when the short piece of coax is
connected, the GPS receives 8 to 9 satellies. When the coax is removed, no satellites are received.

(0 min 45 sec)


This post contains all the information required to reproduce the modification. For those interested, the next post
provides detail on how it works. Finally, I want to caution that this modification only reduces interference
to GPS from the 900 Mhz transmitter via the antenna and may not totally eliminate interference to the GPS
receiver. Other paths include potential propagation on the power and signal lines coming from the
transmitter as well as some GPS receivers internal sensitivity to strong out of band signals.

OMM
Last edited by Old Man Mike; Oct 12, 2008 at 12:29 AM.
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Oct 11, 2008, 11:54 AM
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Old Man Mike's Avatar
Thread OP

Technical Details of Coaxial Notch


The RF engineers out there may recognize this modification as a coaxial notch filter at the GPS L1
frequency of 1575.42 Mhz. Instead of a normal ¼ wave stub, I used a ¾ wave tube so that the same coax
can improve the antenna matching and provide an extended counterpoise. The UT-141 coax has a velocity
factor of 69.4%. Using the formula for a ¾ wave stub:

Length (inches) = 2952/Freq (Mhz) * 3 * Coax Velocity Factor/10

The length was calculated as 3.9" measured from the tip of the coax to near center (acutally at
the 10 mm mark) of the SMA T connector when it connected. Here is a plot taken from a
spectrum analyzer with tracking generator:



As can be seen in the plot, the stub has a loss of only 1 dB at the 915 Mhz video transmit
frequency and a notch of more than 25 dB at the GPS L1 frequency. It also provides a 3 dB
reduction in the 2.4 Ghz band which should also help reduce interference a little if you are
running a 2.4 Ghz R/C receiver on the same platform.

SWR of the Range Video was improved from a very poor 3.5 to 1 down to less than 2 to 1. I
also tested the same coaxial notch filter with a vertical dipole with similar results:

(0 min 42 sec)


The SWR of the near perfect match of the vertical dipole went up slightly when the coaxial
notch filter was added but stayed below 1.5 to 1.

OMM
Last edited by Old Man Mike; Oct 12, 2008 at 12:58 AM.
Oct 11, 2008, 11:58 AM
my karma ranover my dogma
galaxiex's Avatar
Most excellent!

Thank you!
Oct 11, 2008, 12:27 PM
Registered User
Daemon's Avatar
Does the UT-141 coax still contain its center wire, or is it hollow, and
only connected to ground?

ian
Oct 11, 2008, 12:56 PM
Registered User
post some video of your tilting the ship, maybe do a range test.
Quote:
Originally Posted by Old Man Mike
Here is a simple way to improve the performance of the Range Video 900 Mhz Antenna and reduce
interference to GPS receivers. I’ll keep this first post simple and will follow it with a more in depth
discussion on how it works. This modification costs under $15 and will provide the following
improvements:

1) Retunes the antenna to provide a better match to the antenna. The transmitter will run cooler and
put out more power. (SWR improved from 3.5 down to less than 2.0).

2) Reduces interference to GPS receiver by more than 100 times.

3) Improves efficiency of Antenna by a better ground (counterpoise).

4) Reduces interference in the 2.4 Ghz band by a factor of 2 times.

All that is required is a standard SMA “T-connector” and a short piece of coax semi hardline (UT-141) cable
cut from surplus coax jumpers:



The SMA T-connector is available here for $5 and $1 shipping here:

http://cgi.ebay.com/T-connector-SMA-...3286.m20.l1116

The UT-141 coax is available from Down East Microwave. You have to call and ask for the surplus UT-141
coax jumpers. You can cut two of the sections required for this modification from one jumper so that will
give you a spare. Cost is $2 per jumper and $5 shipping. Their website with contact info:

http://www.downeastmicrowave.com/

Here is a picture showing the precise length of cut coax how everything connects:



Finally, the following is a short video showing how well the modification works to reduce interference to a
GPS receiver located 6 inches from the transmitter. You can see that when the short piece of coax is
connected, the GPS receives 8 to 9 satellies. When the coax is removed, no satellites are received.



This contains post all the information required to reproduce the modification. For those interested, the next post
provides detail on how it works. Finally, I want to caution that this modification only reduces interference
to GPS from the 900 Mhz transmitter via the antenna and may not totally eliminate interference to the GPS
receiver. Other paths include potential propagation on the power and signal lines coming from the
transmitter as well as some GPS receivers internal sensitivity to strong out of band signals.

OMM
Oct 11, 2008, 01:17 PM
Registered User
Old Man Mike's Avatar
Thread OP
Quote:
Originally Posted by Daemon
Does the UT-141 coax still contain its center wire, or is it hollow, and
only connected to ground?

ian
Hi Ian,

You just cut the coax to the length shown. The center conductor remains in the coax and you do not short it at the end. After cutting it to the length shown, look at the end of the coax and make sure that it is not shorted to the shield. A Dremel tool makes a nice cut. Just brush off any stray aluminum particles after the cut.

Quote:
Originally Posted by JustinONE
post some video of your tilting the ship, maybe do a range test.
Hi Justin,

Not sure what your request is. I did compare the range of the modification to a vertical dipole and saw no significant difference. Of course I use a diversity receive system with a patch plus vertical so I would not expect to see anything but extreme differences.

Perhaps you are talking about 3D stunts where the antenna's nulls (typically off top and bottom) will cause dropouts when a null is in the direction of the ground receiver. With this modification mounted as I show in the video there is some horizontal radiation from the added coax which will help reduce dropouts when used with diversity ground receiving system containing a patch antenna.

OMM
Last edited by Old Man Mike; Oct 12, 2008 at 12:31 AM.
Oct 11, 2008, 01:42 PM
Questionable judgement
bracky72's Avatar
This looks very promising. Thanks!
Oct 11, 2008, 02:32 PM
Registered User
yes, I meant to have you simulate an acrobatic manuever, but since I also have the Diversity I will probably add a 2.4 and a 900Tx and mount them perpendicular to each other, should reduce the nulls aye?.
Oct 11, 2008, 04:41 PM
Registered User
Quote:
Originally Posted by Old Man Mike
4) Reduces interference in the 2.4 Ghz band by a factor of 2 times.
I like the sound of this. This means to me that I can fly my Spektrum gear further, good to hear.

Does the hyperlink antenna also reduce the 2.4 Ghz interference this much? What gear do you use to test for interference?
Oct 11, 2008, 08:27 PM
Registered User
Daemon's Avatar
Keep in mind, that the graph above represents the response
of this antenna when a constant power signal is swept across the
whole spectrum. It doesn't show the actual frequency spikes
of the 900Mhz Tx when operating on its various channels.
So whether it has a real effect on 2.4Ghz or not is unknown.
But if it was spewing noise at 2.4Ghz, then this antenna mod
would attenuate it by 3dB.

ian
Oct 11, 2008, 09:16 PM
Registered User
Old Man Mike's Avatar
Thread OP
Quote:
Originally Posted by nathanj988
I like the sound of this. This means to me that I can fly my Spektrum gear further, good to hear.

Does the hyperlink antenna also reduce the 2.4 Ghz interference this much? What gear do you use to test for interference?
Quote:
Originally Posted by Daemon
Keep in mind, that the graph above represents the response
of this antenna when a constant power signal is swept across the
whole spectrum. It doesn't show the actual frequency spikes
of the 900Mhz Tx when operating on its various channels.
So whether it has a real effect on 2.4Ghz or not is unknown.
But if it was spewing noise at 2.4Ghz, then this antenna mod
would attenuate it by 3dB.

ian
Ian is correct, in that the spectrum sweep is showing the null points of the coaxial notch with antenna attached. This coaxial notch can be used with almost any 900 Mhz antenna to reduce 900 Mhz noise produced by the transmitter. That was the primary design objective. The 2.4 Ghz interference reduction is only a small side benefit.

If you are not having much interference to GPS but have 2.4 Ghz interference from the 900 Mhz video transmitter, you could lengthen the UT-141 an extra 0.37". This will place the 3rd null at 2.4 Ghz for a 100+ times reduction of interference at 2.4 Ghz. Of course you would then have little if any interference reduction at the GPS frequency.

OMM
Oct 11, 2008, 09:30 PM
Registered User
Daemon's Avatar
I'd still love to see the spectrum analyzer graph of the 900Mhz Tx output
itself, across the same horizontal scale, so we can see where it's
actually spewing noise.

ian
Oct 11, 2008, 10:06 PM
Registered User
Old Man Mike's Avatar
Thread OP
Quote:
Originally Posted by Daemon
I'd still love to see the spectrum analyzer graph of the 900Mhz Tx output
itself, across the same horizontal scale, so we can see where it's
actually spewing noise.

ian
The problem is that most Spectrum analyzers do not have a noise floor low enough to see down to the level where interference can occur. You have to use a special technique of notching out the fundamental and adding a very low noise, high level preamp to see it. Second and third harmonics as well as large spurs are easy to see but they do not appear to be the problem. You can see those spectrum plots here:

http://www.rc-cam.com/forum/index.php?showtopic=2818

The primary interference is mostly likely the noise floor of the transmitter or some very low level spurs which cannot be seen in the above spectrum plots.

OMM
Oct 11, 2008, 11:54 PM
Senior Gravity Checker
OMM,

Great job...I will be trying this shortly. Couple this with the new filter from Mr. Rc-Cam and 900 Mhz will be a non-issue to GPS users.

-Ken
Oct 28, 2008, 06:26 PM
Registered User
great job OMM I have ordered the t connectors and now I need to source the UT 141 cable. I am really hoping this fixed my issues with the GPS cause it's pretty useless as it is now!


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