Friday, August 17, 2007

Antennas and batteries -- Piet le Roux

I am glad to see a lot of technical information and tips in the July issue of SAMAA News. There are however some points where I beg to differ and some points that I would like to ad.

Mr Armitage is a keen advocate of an "antenna outside" installation. I think that a secure antenna installation is important to secure reliability in the long run. To accomplish this you must place it inside the plane. If you bend any type of wire enough times it will break. A wire on the outside is vulnerable to damage and is constantly oscillating in the slipstream. If only a few of its wire strands break, it will cause the same type of noise as vibrating metal to metal contact points. You will be unaware of the deterioration of your antenna because it happens inside the insulation. Most RC pilots never replace their receiver antennas and install it from plane to plane on the outside. An accident waiting to happen! I normally use the inner of a thin cell-phone type coax as antenna wire. When I assemble the plane, depending on its size, I glue the end of the antenna under the vertical fin and keep all the battery and servo leads to one side of the fuselage and route my antenna on the opposite side, securing it with glue and cable ties. On the receiver I leave a short "pig tail" that I solder to the antenna and cover with heat-shrink after I have secured the antenna and receiver. Remember the total length of your antenna must be the same as the original (99cm for JR equipment). Wires and rods running along side the antenna will have little effect on its performance as long as they are not near a resonant length, for 35MHz this is about 2 meters. If it happens that you experience noise from you servo leads use ferrite filters on the leads. Just running the lead thru a ferrite ring will not do much; get the clip-on type that enables you to coil the lead around the ferrite.

If you have a portable transistor radio that covers the shortwave bands, you can utilize it as a noise detector. Tune it to a clear portion of the highest short wave frequency, some goes up to 22MHZ (13 meter band). The level of electrical noise decreases when you increase the frequency, so don't worry that your receiver is receiving below 35MHz; it will just be more sensitive to noise. By placing the antenna of this receiver next to your servos and then moving them you will be able to hear any electrical noise present. The same technique can be used to determent the noise level of a petrol engine's ignition system. Most petrol engine pilots would be shocked if they heard how much noise they generate! The fact of the matter is that any petrol engine plane will have less range with the same 35 or 50 MHZ equipment than a glow powered plane. The 50MHz equipment will be slightly better than the 35MHz equipment, but the best equipment to use with a petrol engine is 2.4 GHz equipment.

The article on batteries was interesting but I do not agree that nickel metal hydride batteries have a lower self discharge as nickel cadmium batteries. NiMH has a much higher self-discharge rate than NiCd. Its self-discharge is 5-10% on the first day, and stabilizes around 0.5-1% per day at room temperature.The rate is strongly affected by the temperature at which the batteries are stored with cooler storage temperatures leading to a slower discharge rate and longer battery life. So it is important that you charge your NiMh batteries the night before you go flying and not after you have been flying! A new type of nickel metal hydride battery was introduced in 2006 that claims to reduce self-discharge, and therefore lengthen shelf life. By using a new separator, manufacturers claim between 70 to 85% of capacity is retained after one year, when stored at 20 degrees Celsius . These cells are marketed as "ready-to-use" rechargeables, and are targeted towards the digital camera market, where the equipment is only used a few times a year. Users must be aware that a normal NiMh battery deteriorates during long time storage. This problem can be solved by charging and discharging the battery several times before use. One of our CRF members recently bought a new hi-capacity NiMh pack to replace his suspect pack. He then charged it only once with his "smart" Swallow charger. During his second flight of the day the PCM receiver went into safe-mode, due to low battery voltage while the plane was in a dive, his beautiful Genesis was demolished.

Lekker vlieg
Piet Le Roux, Bloemfontein.

(Originally posted on the SARFLY list)

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