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Sep 15, 2011, 06:36 PM
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arocholl's Avatar
Mini-HowTo

Improve your antenna with RF Explorer: Simple procedure



Simple procedure to improve your Antenna using RF Explorer

As described in ref[1]:

Antennas may seem to be a mystical art. Unlike many electronic devices, any change in nearby materials or dimensions can affect antenna performance. Trying to build a published design does not guarantee results. Testing an antenna design is necessary, tuning is usually required, and there are pitfalls along the way.

In this short article we will see how to use deceptively simple techniques to finetune your antenna without being an expert, just by using RF Explorer but can be applied to any Spectrum Analyzer (SA). You don’t need to understand concepts such as VSWR, return loss or the like.
Let’s agree first on a basic idea: whatever a communication link does, what we always want is the Receiver to get the best possible signal/noise ratio. Assuming noise being a constant factor here, we are left with the best possible signal level in the receiver. That is, we want the maximum power received.


There are many different antenna designs, see for instance ref [5]. In any case, the main goal is always this:
  • For a transmitter (TX) it should transmit the maximum possible power
  • For a receiver (RX) it should receive the maximum possible power
Based on the reciprocity principle [ref 4], for low power systems we can measure any antenna as either a RX or TX, whatever is easier, regardless their actual function in the link. That gives us the opportunity to use RF Explorer very easily to measure TX and RX antennas equally well.

The transmitter/receiver can be anything, your Video link or your LRS link. As long as you have a RF Explorer of the right frequency.

Using RF Explorer as a receiver – simple steps

All RF Explorer models can be used as a receiver. In fact, a SA is just a sophisticated receiver that can provide accurate feedback on the link quality.

You must run all measurement in an environment where the EMI noise is low so you can easily differentiate your transmission from other RF sources. You should switch off all cell phones, Bluetooth devices, RF transmitters etc that are not part of the test and may end up confusing you in the results you get.

TX should be a few wavelengths distant from the RX (for 400MHz or higher frequencies, 10mts would be the minimum distance to use). Once tests are started, the distance has to be kept constant among tests set. You can start another set of tests anytime by modifying the environment any way you want, but you cannot compare these test sets between.

Let’s assume you have designed and want to fine-tune your Video RX antenna. Let’s also assume it is a quarter wavelength whip antenna.

To test it just plug it in RF Explorer and follow the next steps:
  • Configure RF Explorer to the required span so you can capture the frequency range of the transmitter. For video this is typically in the range of 20MHz. Remember you should not change span settings or other parameters once you start testing your antenna, keep it constant across all the testing.
  • Set the Calculator=Max and the Iterations=20 in the ATTENUATOR MENU
  • Keep RF Explorer and the TX out of the influence of any large metal such a car. If possible, take the measurement in the very same environment this will run (i.e. your flying field). This will minimize the impact of multipath effect. If you test this in a closed room the bouncing signals will produce crest and valleys and will distort your actual measurement.
  • Switch your TX and read what you get in RF Explorer. Let’s say you get -35dBm.
  • Try several antenna orientations (both at TX and RX) till you get the best possible read. Sometimes using a TX or RX directional antenna the best orientation may not be the one you expect. In any case you do want the max power so try all combinations. Let’s say you get now -32dBm peak reading.
  • Start now manipulating your antenna parameters. As we have designed a whip antenna, the most obvious parameter to change is length. By making an antenna shorter or longer, you modify its impedance and efficiency, but remember we always want the max power read. So shorten or extend the antenna length by 2mm at a time and read again. If you are getting lower and lower reads after certain point, then you got the best possible antenna for your setup in the length where you got the maximum read. See below an example


From this you can see apparently the best length is about 33mm, close but larger than 30mm predicted by theory.

Therefore a good technique to tune a whip antenna is to build it 20% larger than it should and start above exercise till you get the best length.

With this simple technique you can get easily 3dB or more from a simple antenna design. This is equivalent to doubling the TX power…

Some antennas are not as simple design as a whip, and you may have additional freedom to manipulate the geometry in further ways. For instance , a circular polarized antenna may need changes to get the best possible reads in all antenna orientation, and a directional yagi may benefit from offsets in the director and reflector elements.

Still the principle is the same: modify your setup till you get the best dBm read.

Making it more complex

The above technique works when the antenna is acceptably close to its ideal setup. In other words, if the antenna is completely wrong for a RX or TX design it may take forever to tune it with above technique for any complex antenna with several geometric level of freedom to change.

As an example of this limitation, the RF Explorer has 50 ohm impedance as most RF instrument and normalized devices. If you have a TX or RX which requires a 100ohm antenna, then what is best for RF Explorer may not be best for your setup. To use RF Explorer you will need a impedance match from 50ohm to 100ohm.

Another option is to measure VSWR, but that should be taken as a complement to above simple procedure, not as a substitute. Regardless what VSWR say, you want max dBm received not perfect impedance match only. As a matter of fact, a pure 50ohm resistor will be perfectly tuned but will be a null transmitter.

In many cases, a larger antenna is a more efficient one regardless having higher VSWR than a shorter one.

VSWR becomes critical at high power transmitters when hundreds of watts reaches the antenna. In these circumstances, a high VSWR will reflect a lot of this power back to the transmitter and that may damage the power circuits.

In any case, VSWR can be measured with RF Explorer: The required technique to tune non-50ohm designs requires a directional coupler and two RF Explorers, one as a RF Generator and another one as Spectrum Analyzer to measure the amount of signal reflected and thus tune the antenna for your specific non-50ohm system. You can get a good description of such a technique in ref [2].

I will write a specific future article on VSWR measurement once I can offer a wideband tracking generator module for RF Explorer with a “certified” directional coupler set.

References
[1] Antennas for Low Power Wireless Applications by Kent Smith - http://www.rfm.com/products/apnotes/antenna.pdf
[2] Antenna Measurements by ICTP http://wireless.ictp.it/handbook/C6.pdf
[3] Antenna Measurement by Wikipedia http://en.wikipedia.org/wiki/Antenna_measurement
[4] Reciprocity by Wikipedia http://en.wikipedia.org/wiki/Recipro...romagnetism%29
[5] Antenna Construction by ICTP http://wireless.ictp.it/handbook/C5.pdf

Updates

2011-09-16 - References
Last edited by arocholl; Sep 17, 2011 at 10:32 AM. Reason: see [Updates] section
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Sep 15, 2011, 06:44 PM
Registered User
I'm unsure how you can do much with this test for anything but an omni whip that is disposable.

For example, without destroying the antenna, how can you shorten past the point of no return any CP or patch design.
Sep 15, 2011, 06:47 PM
Electric powered
arocholl's Avatar
Quote:
Originally Posted by nyc863
I'm unsure how you can do much with this test for anything but an omni whip that is disposable.

For example, without destroying the antenna, how can you shorten past the point of no return any CP or patch design.
Absolutely. This can only be done in a whip that can be shortened or extended. Other types of antenna can be modified in geometry in other ways, think on a patch antenna you may need to try different dielectric materials and distances, different plate size, different feed point, etc.
Sep 17, 2011, 02:59 AM
Registered User
jdm66's Avatar
Hello,

Thank's a lot for "How to"

Have you a procedure to tune a Biquad, please?

Thank's for support

jdm66
Sep 17, 2011, 05:30 AM
Registered User
Toysrme's Avatar
& seemingly be locked to a frequency range RFE seems to be stuck in, OOOOOOOOR you could spend 6+6 12us for priority + return shipping to alex (or any of several others here) & give'em a couple bucks for their trouble.
Sep 17, 2011, 09:07 AM
Registered User
Congratulations, Ariel, for your post.
Thank you also for the references.

It is a pity your Explorer is not available in 1.3Ghz band.
Sep 17, 2011, 10:02 AM
Registered User
jalves's Avatar
Nice article.
Sep 17, 2011, 12:26 PM
Questionable judgement
bracky72's Avatar
Quote:
Originally Posted by Toysrme
& seemingly be locked to a frequency range RFE seems to be stuck in, OOOOOOOOR you could spend 6+6 12us for priority + return shipping to alex (or any of several others here) & give'em a couple bucks for their trouble.

Good thinking. Why do it yourself when you can pay another to do it! The new American way.
Sep 18, 2011, 06:27 AM
Electric powered
arocholl's Avatar
Quote:
Originally Posted by jdm66
Have you a procedure to tune a Biquad, please?
Not a specific procedure but a generic one: if you build the Biquad yourself, you can play with the different geometrical parameters to get the best dBm read.
Sep 18, 2011, 05:15 PM
Electric powered
arocholl's Avatar
Quote:
Originally Posted by 3frcg
Congratulations, Ariel, for your post.
Thank you also for the references.

It is a pity your Explorer is not available in 1.3Ghz band.
Thanks, I hope it is useful as a starting point. Finetuning a complex antenna may take a lot of time, but at least the instrument is available.

I am working to offer 1.2/1.3GHz band support...
Apr 18, 2014, 10:09 AM
KISS
hefest's Avatar
Maybe simple question, but when measuring performance of particular antenna do you look at single power peak or average on complete 20mhz span?
i,m asking this because video is transmitted in 20mhz wide band with several peaks.

interesting thing is that if for example video is generated only by OSD without camera attached graph has only one peak but with more power.
Apr 18, 2014, 10:57 AM
Electric powered
arocholl's Avatar
Yes, that is a good question - a simple peak means no modulation, single CW being transmitted (there is no video signal to modulate) therefore all energy is concentrated in a single point in frequency. When there is a modulated channel, the power spreads accross the channel width.

From antenna tuning perspective, it is always best to use a CW - just because it is easier to measure. In any case, as long as you compare equivalent setup (either modulated or CW) you can use any option.
Apr 19, 2014, 05:09 PM
UAV Integrations
Excellent how-to article! Good job.
Apr 20, 2014, 02:59 AM
Registered User
wazoo22's Avatar
Thanks for this. Very useful. Will do this on my TSLRS dipole today!
Sep 08, 2014, 06:54 PM
question:
I'm trying to test a 1W OrangeRX UHF Tx and RX. I'm worried about overloading the RF Explorer with the power from the TX (is actually in the 300-600mW range, depending on input voltage). So, I'd prefer to use a 30dB attenuator that I have.

question:
When testing with an attenuator, is it preferable to run the attenuator connected to the RF Explorer or to the transmitter? Or, does it make no difference either way?


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