Whip it is it? - RF Cafe Forums

 Post subject: Whip it is it?

Posted: Mon Sep 26, 2005 7:36 pm 

I am involved with a RC car club. A typical RC car consist of a battery, a few servo's for steering and control functions, and a receiver for accepting input signals sent by the users hand held control pad. The receiver in this case operates in the 27 MHz band. Now here is my question. I have noted a few things that seem inconsistent with standard text book design practices. The power leads from the battery to the receiver are very long and of a small gauge. Ground connections are only from the battery to the receiver. There is no ground connection to the metal chassis and this puzzles me. Should there not be a ground plane for the antenna? The antenna was the most puzzling of all. It is a piece of wire with a length of 18 inches. This length would be something like a 1/18th wave. I looked up as much info as I could find on the subject of antenna's to see if I could determine what type of antenna it is. So what kind of antenna would a fractional wave piece of wire with no ground plane be? Would grounding the metal chassis to the battery ground improve performance? Is there a practical method to test and verify any modifications?
I appreciate any help you can provide, and thank you for your time.

 Post subject:

Posted: Tue Sep 27, 2005 9:13 am 

I think the antenna you are describing is one missed by many textbooks. It's the "ultra cheap antenna". I think at that freq. it is impractical to have a proper tuned antenna without the company spending a ton of money on r&d. I think they feel the range is adequate as is.

Your best bet would be to experiment with different designs and see which work best. Test equip, such as a network analyzer might help, but you really don't know the characteristics of the receiver itself.

 Post subject: The Cheapo antenna debockle

Posted: Thu Sep 29, 2005 3:40 am 

Well I pretty much realized the cheapo approach that they use. Still wonder though If I were to tie the Battery ground to the chassis, would my receiver performance improve? Also would a Base load help much? If so how would one go about determining the proper load to use.

 Post subject:

Posted: Thu Sep 29, 2005 8:09 am 

Greetings attenuator:

You appear to have at least a Ham's knowledge of antennas (not to say that a Ham's knowledge is trivial), so why don't you try out the modifications you mention and report back to us with the results? Experimental confirmation is where the rubber meets the road (pun intended - RC car, get it? ).

I fly RC airplanes and helicopters, and before every flight, I have the system turned on and walk away from the model (with someone holding it for safety) with the transmitter antenna collapsed and verify that the operational range is sufficient both with and without the engine or motor running. Electric-powered craft are particularly harsh on RC receivers (although the new breed of brushless motors is minimizing that problem).

My electric-powered helicopter, operating at 72 MHz, has a wire receiver antenna that is wrapped several times around one of the landing skids. It surely violates every rule of antenna efficiency, yet for the distances at which the small heli is operated, the controllable range is fine. The same is probably true for your car example.

One difference with RC cars, though, is that people are usually racing them while standing in close proximity with as many as 8-10 other guys who could be just one channel away. In that case, the bleed-over from a transmitter on an adjacent channel could cause a poorly tuned antenna to be susceptible to interference.

Anyway, I and others on this forum would surely be interested in your results.

_________________
- Kirt Blattenberger
RF Cafe Progenitor & Webmaster

 Post subject:

Posted: Fri Sep 30, 2005 10:35 pm 

I appreciate your input Kirt. The only involvement with HAM that I ever had is on holidays when I would eat it. Actually my background is Digital Hardware design. My last position was at Qualcomm. I left because I was tring to make a career change. Big mistake.
Anyways I made the following mods, on my RC car. I shortened the power cables as much as I could. The tiny molex plugs impose a restriction on wire size, so I opened my receiver and soldered a large gauge wire from the ground pin on the receiver to a brass plate that I bolted to the top cover of the radio box. I then soldered another wire to the brass plate with the other end terminating to the aluminum chassis through a lug and was held with a nut and bolt. I had to use a dremel rotary tool to clear away the paint around the nut to ensure a good ground connection. Prior to making this change I had noted that the car servos were twitchy and would jitter whenever I touched any metal part on the car, most notably the car gas engine. After grounding the electronics the above mentioned problem was eliminated. My theory as to why is this. Without a ground plane the electromagnetic waves couple to the metal parts of the car such as the engine. This would then make the engine act as an emitter and a source of EMI. Other things I think was that the ground plane acts as a mirror of the whip, and help the whip antenna appear closer to the correct length. I will be going to a RC race this weekend. So I will be able to determine if problems with EMI occur. I really don't see anyway real way that I can test this, as all I have is a scope. So I am basing my assumptions on the fact that by design the performance should be better. Let me know what you think.

 Post subject:

Posted: Sun Mar 05, 2006 7:03 pm 

Hi attenuator,

Did you have a look at the base of the antenna to see if there isn't any coil attached to it?

If yes this antenna is a short whip. It's only 1/18 wave length long but the coil is here to cancel the capacitive effect due to short length. Of course the efficiency is very poor compared to a 1/4 wave antenna.