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These are the results of the first simple test we did over some stock antennas and a hand made one using a 2.4 Ghz SWR meter.

To do it the first thing was to check the accuracy of the transmitters, a 3W and a 0.7 W unit, using a dummy load instead of any kind of antenna.

Dummy%20load.jpg

Next, we place a rubber duck antena , the stock one supplied in most of the TX-RX combos. On the left side the device shows the output power (marking 7) and the reflected on the right side (close to 0 value). The cross point of both needles gives the stationary level value for the antena SWR (1.1) That´s a good level and looks like a well matched antenna. This kind of antenna shows very little if none differences when switching between the available frequencyes of the TX. Have tested three different ones of the same model with inappreciable difference from each other. One strange thing we have noticed is that the rubbered cover of the antenna has a big influence. Removing the rubber cover and testing now the "naked" antena ,the reflected needle goes to a value close to "2" That means a total reduction in output of +-30%

Rub%20duck.jpg

The next antenna was a 5 dbi. (in fact we tested also 3 different models of the same antena) This antena has a inner wire cut at a certain longitude. Working at 2.4 Ghz you need to be 1mm short or long to have the antenna out of syntony. The three antenas showed different values when working at the same frequency. Some are shorter than needed and some are longer than needed. They give better output level (0.8 W) but having the reflected value higher the total gain is quite reduced and some powerful emitters can be damaged due to it. Average SWR value: 1.75 These antennas need some work to get them matched and are also affected by their own rubber cover, so dont try to use it without the cover.

5dbi.jpg

Another popular antena is the 8 dbi circular patch

We normally use it to receive and not for transmitting but a matched antenna needs to work in both situations.

In this case the forward power is 0.6 w only showing a reflected value of 0.2 that means a SWR of 1.45 aprox. Not the best values but not the worst we saw today.

Circ%20Patch.jpg

Finally my prefered one. This antenna was hand made by Julian Cox (Palrotor-Flightpower) years ago with no other help except a digital caliber. Sure Mr RC-Cam can remember this old post. http://runryder.com/helicopter/t49030p1

Must say I commonly use it since these days with great results in the helicopter video system and was succesfully tested when used as areceiver antenna when flying fixed wings airplanes at distances where others started showing interferences. Today we have had the opportunity of checking how well or bad it was done and here are the results.

This first picture shows a quite high reflected level (2) and a total SWR of (3.5).

JC%20org.jpg

We started cutting the inner wire 0.5 mm at a time and finished here after cutting a total lengh of 3.mm. In this case the antenna has a pigtail that has been matched too at the same time. The right way to do it is to match a pigtail at certain longitude (because in 2.4 ghz it´s quite important) and then add it to a matched antenna. That was not possible here but even so Julian made a good job here. It´s quite difficult if not impossible to fine tune it without this equipment.

JC%20mat.jpg

What we tested here has no relation with gain values or radiation patterns. We see here if an antenna reflects any amount of the available output power or on the other side it´s well matched and it radiates all the available power increasing therefore the total distance and reducing the output transistor stress of your TX.

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Nice work - thanks for the report. It would be very interesting to test what the experienced range differences were for antennas that were tweaked to perfection (like the 1mm trims you mentioned). Big VSWR's such as 2.5:1 result in less than -1dB of RF loss. So, I'm not sure anyone would recognize a significant range improvement after some tweaks. But practical experience is king in our game.

As you mentioned, big VSWR's can stress RF PA drivers. But our video Tx's tend to use RF levels of just a few hundred mW's and I have not heard of anyone that has done any damage to their Tx with VSWR abuse (thank goodness). It seems to me that your massive 3W rig is one that might not like a high VSRW, so your tweaks would not be a bad idea for it. :)

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1 db lost could be significative or not deppending on the rest of the parameters, total gain of the system, total output and expected usable range. I.e. those of you in the states under the 10 mw law expecting a big range for FPV could find this simple 1db the difference between an usable system or a P.O.S. or others trying to achieve the limit using omni antenas.

Any way the intention of this test was to see how fine the commercial antennas are. Will see what the field test has to say. At least I know what frequency to choose to get the best for each type in every test.

Have fun.

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Anyone knows what kind of material the manufactures use to build the rubber cover of the antennas. It´s amazing how this covers affects them. Looks like they have some ammount of metalic component but the antennas are tunned having this cover placed and goes out of tunning removing it. The same way, placing this cover over a the hand made antena nicely tuned makes reflected needle go to a 2 value. :o When trying the same with a simple plastic cover it has no effect. Is possible the stock cover is reducing the signal in anyway?

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Is possible the stock cover is reducing the signal in anyway?

A poor plastic choice would affect range. But, the only way to really know is to measure the radiated power (rather than VSWR) or by performing a careful range comparison test.

Please keep in mind that VSWR measurements can be deceiving and radiated RF power measurements will offer the best data. The required test equipment is expensive, so field range tests would probably be a practical solution.

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