There you are!

You can "tune" the match by very slightly adjusting the spacing between the end of the two halves of the dipole. You can also move the feed point points in and out the elements. There are also a variety of matching schemes that can be used to interface between the cable and the dipole.

Here is a (very) good starting point: http://w2du.com/

Follow the link to Reflections 2.
 

Nice link!  The author points out a lot of misinformation that is out there regarding antennas.  Some light reading for a cold winter's night!  

John D

The dipole lengths and the balun are more important that the spacing for proper tuning. Suggest not having the spacing extremely small.

 

Absolutely agree that the element lengths are the very first thing to tweak. The calculated lengths are typically a good starting point.

But moving around the Smith chart to match is (over-simplification warning) is essentially a two dimensional problem. So to achieve perfect match often requires another variable. That's where all the various matching schemes enter. Commercial UHF antennas use schemes such as "gamma match".

Baluns in themselves are just transformers. Often 1:1. For that reason their contribution to matching is often via indirect secondary effects.

The mentioned frequency of 910 MHz implies a low power data link. Commercial antennas (such as moderate gain Yagis) are available fairly cheap. Like under $50 from companies such as L-com.

It's also well worth mentioning that a perfect match should not be the main goal. There's a point where close enough is much better than perfect.


 

A perfect match isn't necessary.  VE1BLL nailed it.  This is for short range video (NTSC) transmission.  The problem is that the antenna I need isn't commercially available due to space and weight requirements, so I'm on my own for the system here.  I'm receiving with a patch antenna, but my transmitter must be omni-directional and low gain.

If I can get an SWR of 2:1 or better it should work.  The real problem I'm having is with interference.  I'm getting a lot of static and the transmission is unusable past 1200 meters which is terrible for 500 mW.  I'm also having trouble with a nearby GPS, which can't get a lock (I'm guessing the reason is spurious emissions at the transmitter).  I was thinking about using a balun or perhaps a low pass filter, but this unit will be in close proximity to several rotating motors which is making things even more challenging.  Changing the motors from brushed DC to 3 phase brushless DC drives seems to have cut down on the interference due to the motors.

This is for a mobile surveillance robot.  It is a complete transmission nightmare.  I have many of the problems ironed out, but still working on a few.

Thanks for the help.

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If it is broken, fix it.  If it isn't broken, I'll soon fix that.

Some suggestions:

On the robot, you can still have a medium gain antenna and omni coverage. By stacking up a colinear array you can concentrate the signal on the horizon, but you probably have height issue that would preclude that approach.

You could use four quadrant anteannas and command them remotely.

You could increase the gain of the receiving antenna. Even a moderately high gain antenna still has a fairly wide beamwidth. Plus the operator can aim it as required.

Add a very low noise figure, moderate gain LNA to the receiving system, right at the antenna. This is easy and might make a big difference if weak signals are the main issue.

There are much higher power transmitters available. For example, RF-links.com. But might require licensing.

1200m range for 500mW isn'd bad considering that it is video. A wideband signal like video requires a much higher power to overcome the noise included in a wideband channel. The fact that you're getting almost 1.2km with a half watt indicates that you're doing a lot of things right.

A balun is simply a BALanced to UNbalanced transformer. It solves a problem that most installations do not have. And it naturally adds a wee bit of loss. The best balun (if required, not likely) might be several turns in the feedline to block common mode currents.

Position the GPS antenna as far as possible from the transmit antenna. It may be simple overload. Use a better quality GPS. Add a filter to the transmitter if you can prove it is spurious emissions. Spurs would block the GPS at mush longer distances. Overload will disappear with relatively small increases in spacing.

Modern robots might be moving to Internet Protocol.


 

VE1Bll, thanks for the suggestions.  I will look into other antennas on the transmit side.  Right now I'm using an 8 dbi patch antenna for the recieving side.  I was thinking about going to the Yagi except the beam appeared a bit narrow.  I don't know much about quads, but I'll look into that one.

Right now this is still in the experimental stages.  We were having trouble with the GPS system if we increase transmiter power to 1 Watt even with a commercial antenna.  The best spacing I can get is about 30 cm, not much.  I went with 900 MHz because of all of the internet routers are 2.4 GHz as well as many cordless telephones.  900 MHz band appears to be used infrequently.

Right now I'm ironing out the issues above ground in line of sight.  Once this thing goes underground/underwater I'm sure I'll be having all sorts of new problems.  Luckily the distance underground will be very close, probably less than 30 meters.  I'm well aware that the GPS will be useless underground.  I might just eliminate it and go to 1 Watt.

Thanks again!

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If it is broken, fix it.  If it isn't broken, I'll soon fix that.

I'd suggest a monopole on a ground plane. Very easy to make.
Set height at 0.25*11,803/fmHz in inches. No balun. Make the ground plane twice the height of the antenna ballpark, larger is ok too. Trim the height for perfect VSWR match.


 

Keep in mind that the quoted beamwidth of an antenna is typically for the -3dB points relative to the boresight. But a 15 dBi antenna will obviously have more gain (than the 8 dBi antenna) even down to its -7dB points. Thus the angle over which it has more gain is wider than might be indicated by the quoted -3dB beamwidth.

Furthermore, the added range typically lessens the apparent angle of movement of the target. And if the target is much closer so that its apparent azimuth changes quickly, then you don't need the antenna gain anyway (because the robot just ran over your foot - so to speak).

Worse comes to worse, add a coaxial switch and provide both antenna options at the receiver. At least until you get past the prototype stage.

For the GPS interference, that's just an implementation problem. GPS is built into mobile phones, even the iPhone (for example) that has 'apps' that downloads maps on the fly over the 3G data network even while maintaining GPS lock. All packed within a couple of cubic inches volume.