Spektrum Receiver Antenna Modification

This is the modified Spektrum receiver I have been using with my Futaba 3VCS this season.


On problem when using this receiver on an IOM is that the stock antenna is only 200mm long. Some have tried having the Rx in the rc-pot and running the antenna only a short bit vertical above the pot lid(low enough so that it won't hit the main boom). But this has not given enough range.
So I wanted to extend the antenna and have it on the aft deck. Now this unit works at 2.4 GHz and at these freqencies the signal would get seriously attenuated using anything other than a purpose-made coaxial cable for 2.4 GHz. However, I also wanted to have the receiver removable and replaceable in an emergency... so I removed the plastic cover of the Rx and soldered on this SMA connector. Note that the ground and signal pins are not touching eachother although it might look like that !

"The good thing about standards is that there are so many to choose from"
-- Unkown
There a number of connectors that are called SMA, but nevertheless are different and uncompatible. Either the plug or socket can have the pin end of the connector, and some narrowminded device producers have chosen a standard with reverse threads on the connectors. So be sure to check for compatibility when buying SMA connectors...

This is the coax cable that takes the signal from the RC-pot to the aft deck. The deck connector is a wall-mount type. There are a number of grades of coaxial cable. Choose one that is narrow and has a reasonable attenuation at 2.4 GHz. If I remember correctly this cable is called RG-316 with an outer diameter of 2.3 mm. It has slightly less attenuation than the "normal" narrow coax, RG-174, found in most places.

Here is the deck connector mounted far enough back so there is plenty of room for the no1 boom to swing by. This picture shows the 2 dBi omnidirectional antenna that I have used most during the season.
Look long enough at this picture and you will be hypnotized by the deck pattern to think only positive thoughts about Texalium ! 🙂

Since the 2.4 GHz ISM band is used by lots and lots of other equipment there are plenty of different antennas available. From left to right: 5dBi omni, 2 dBi omni, and 0 dBi omni. The 2 dBi which I have been using (in the middle) is about 95 mm long - to give you a sense of scale. The 5 dBi is a little big and heavy to use on the boat but Rob Guyatt has used one on the Tx module - he says it improves the range a bit but not very much.

First I bought some antennas and cable from Wimo in Germany but then I also found a Finnish company, Elcard (www.elcard.fi), selling this kind of stuff.

During the season I have been fairly happy with this arrangement. If I remember to keep the Tx antenna vertical and not stand behind very many people or large metal objects then the range is good. The precision and jitter etc. properties are as good as PCM. The modification described here is a little hard to do so I think I will go for something simpler next time. Rob Guyatt has used just a piece of coaxial cable to extend the stock antenna well above deck. Everything soldered together but threaded above deck through a tube allowing replacement if neccessary.

The Rx draws a little more current than a PCM receiver does but I have not noticed a difference. The Tx module draws less current than the 27-40MHz modules do and I consistently get around 7 hours of uptime on the Futaba 3VCS using an 8-pack of about three year old 1600 mAh NiMHs.

Interestingly, I note that spektrum has quite recently released a complete 2-stick system, the DX 6,using this technology. It's labeled as a park-flyer system and sells for a reasonable 199 usd. How long before anyone tries this on an IOM ??