recieving antenna.


When I used to play around with CB radio we matched the physical antenna
length to the frequency we were working i.e 27mhz was 11.1 metres so a
1/4 antenna was about nine feet, would it improve the reception and in
turn the range oof a transmitter if tranny and receiver had 1/4 wave
antenna? they would be easy enough to manufacture with the one in the
a/c being coil wound and hidden in the rear fuselage,
regards, Terry
Reply to
Terence Lynock (MSW)
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The transmitter IS already tuned to a quarter wave by bottom loading.
The receiver aerial is not tuned at all and feeds such a completely mismatched impedance that tuning it would have little effect..and anyway its pretty hard to complete the 'earth' part of a 1/4 wave whip in a plane.:-)
A dipole would be a better choice, but they are very directional.
The reality is that in an aircraft, you want an aerial that won;t be massively detuned by running near pushrods and servos, so its just a random bit of wire that is 'waved in the vicinity'* of the receiver front end.
Its easy enough to build a receiver that is sensitive enough to be fully saturated by the other electrical noise around, without resorting to tuning the aerial anyway.
*
its usually coupled in via a very small capacitor or s low turn on a tuned coil. > regards, Terry
Reply to
The Natural Philosopher
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As Terry pointed out, it is better to have as strong as possible a signal emitted by the transmitter with the receiver just able to receive said signal and no more.
If you have ever operated a high quality CB set, a good ham receiver or a good broadcast band receiver, you will notice that they have a knob labeled RF or sensitivity. This basically allows you to attenuate the incoming signal so that other signals will be suppressed. This assumes that the signal that you want to receive is the strongest signal present to your receiver's input.
Some model grade receivers utilize a circuit called an AGC (automatic gain control). The latter technique is so effective that JR has been building and selling single conversion receivers for a long time and utilizing the AGC to suppress other, weaker, signals. No one scheme of design is satisfactory for every possible situation that one can encounter, but JR's design has worked very well for the majority of model uses.
Ed Cregger
Reply to
Ed Cregger
Dunno how it is on your side of the pond, but in the US where the R/C allocation is in the 72 MHz band, Rx antennae are generally pretty close to 1/4 wavelength, about 1 meter (figures to 1.04 M at 72 MHz with c=3.0xE8 for approximation).
Abel
Reply to
Abel Pranger
So they use little or no bottom loading.
Unsurprisingly the sets on sale here on 27 MHz,35MHz, and 40MHz use exactly the same aerial as you do...Just different electronics.
The receivers likewise.
Transmitter aerials are tuned, as it gets more power into the air so to speak, and the conditions - being held by the pilot with his feet plonked on the ground - make them amenable to it.
Receiver aerials are not tuned *as aerials*..though the better sets do have a tuning circuit before the mixer, and sometimes a second one if they have an RF stage..the main design goal is to get enough signal in without tuning so critical that installation issues will throw the whole thing off tune.
The way to achieve that is to ensure a huge mismatch on the aerial to receiver coupling..tests have shown that the aerial length can be varied by very large amounts without making a huge impact on the range.
Reply to
The Natural Philosopher
Once you have enough signal, anything extra has to be attenuated anyway to prevent distortion.
Ed Cregger
Reply to
Ed Cregger
I would be shocked if the receivers did not have an AGC circuit that feeds back from one of the IF stages to reduce the gain of the first RF stage. They could not operate reliably without it.
If one had the patience, they could install a small transmatch between the receiver and the antennae and tune the specific installation. It wouldn't make much difference, though, except in rare fringe cases where the airplane got a little too far away. Anyone that flies their planes that far away on a regular basis is going to loose one sooner or later, with or without a tuned antennae in the receiver.
I have designed and built several transmatches and spent quite a bit of time experimenting with them. I can transmit into a bedspring or an iron fence, and the transmitter sees perfect matching impedance. However, most of the energy is dissapated by the transmatch, and little makes it to the bedspring, fence, etc....that is the problem with matching networks. If the anennae is badly mismatched, most of the energy goes into the matching network, and reception and power out is poor.
Reply to
Ook
Hi Terrence, IMHO it wouldn't really matter because I don't know of any brand name rig that won't give you OOS range. Therefore any increase in range becomes moot. However, my layman's intuition is telling me that some sort of general tuning exists in both Tx and Rx antennae because of the explicit warnings to insure that the Tx is fully extended and that you do not cut excess Rx antenna.
Reply to
Ed Forsythe
True for a TX, but the RX situation is more complex. Tests on a GWS receiver revealed it did best with about 1.5 meters of antenna, wasn't bad on anything between 0.5 and two meters, and just lost range progressively and smoothly below 0.5 meters.
That is simply showing that its tuning (if it has any at all), is phenomenally broad. The impact of reducing the aerial size is far more consistent with simple loss of area covered by it.
IIRC range is simply proportional to the length of the aerial, tuning issues aside.
Reply to
The Natural Philosopher
The message from The Natural Philosopher contains these words:
This is what I was getting at in a way in that if you fittes a n ine foot open coil antenna inside the fuselage which would only be nine feet of copper wire wrapped around a thin plastic tube would it help increase reception and by way of this improve things like aileron flutter etc, it is different with CB in that the receiving antenna is also the transmitting antenna so has to be specifically tuned to the fequency being worked but I know the bigger the antenna the better the reception and wondered if it was also the case with r/c aircraft,
regards, Terry
Reply to
Terence Lynock (CSD)
The only reason we have an external antenna on a trany is because of the stupid FCC reg that says the signal is supposed to be vertically polarized. Then no one holds the trany so that such a signal is transmitted in violation of the law I suppose. You can fly a JR trany just fine with the antenna unscrewed and not even attached. More then adequate range in the air. I have flown Futaba tranys with the plane pretty far out with the antenna down all the way and seen no real problems. I can only conclude that in general we have way more signal then we need if the antenna is installed properly. So why waste time fooling with the rx antenna when it is already more then sensitive enough to do the job?
I even heard of one over water range check on a slow stable model that almost flew itself. Trany was on shore. A chase boat followed the plane and via marine radio told the pilot what to do. They were miles out and still had control. I will admit over water is a best case and over land you likely would not get out miles. But over land you never run out of range as long as you can see the plane unless someone turns on a second trany on your frequency. In that case more sensitivity will make no difference anyhow.
Reply to
bm459
No ..a helix still only covers the same area as a piece of wire about the same length as the helix.
From rusty memory the energy is 'collected' over an area that is about that of a circle constructed in the plane of the total aerial, and of the dameter equal to the largest linear dimension.
I.e. folding a long aerial in half effectively is one quarter the area, and therefore one half the range,
Reply to
The Natural Philosopher
No matter what snipped-for-privacy@scn.org says do NOT fly with the TX ant. down. I have done this and it took an Uproar from(I guess I took it from myself). I do not mean to be rude but I certainly do not want anyone to get the idea that flying with the Ant. down is going to work. mk
Reply to
MJKolodziej
No that's not quite right, the electrical aperture (which is probably the most important antenna parameter for RC work) is not the same as the physical aperture.
It's generally smaller, the classic case being a microwave horn (and one that is easy to visulize) the electrical aperture is usually only 75-90% of the physical aperture.
But, OTOH, a narrow-band tuned helix will give MUCH greater range than a straight wire of the same length, try it.
However, we have a large margin with modern RC gear, I have often flown a model (with care, at the right height, upwind and an engine failsafe,-- for all the doom merchants) with no antenna and or reduced to one section with good results.
Antennas are rather complex, and have many parameters to consider, nearly all of which inter-relate. J Krauses' 3rd edition of his famous text, "Antennas" is more readable than the 1st and 2nd Eds and is an interesting text, if one wants to get into them.
Reply to
Barry Lennox
The message from tux snipped-for-privacy@nowhere.at-all.net contains these words:
Sounds like the same as CB rigs where you have a SWR (Standing Wave Ratio) to match to your operating frequency, the idea being you lengthen or shorten the antenna until you have a SWR as close to 1:1 when transmitting as possible, disconnect the antenna and key up and you will hit 10:1 and after about 30 seconds watch the smoke start to rise,
regards, Terry
Reply to
Terence Lynock (MSW)
Basically you are on target..
However it has been found not to be so clever to make transmitters that burn out when the aerial is retracted - most modern designs have enough margin to survive, tho mostly they do get hotter.
Reply to
The Natural Philosopher
Not really a myth. However, most r/c equipment does not put out enough power to damage the output amp. Any ham operators out here? Know what happens when you tune up your transmitter, disconnect the antennae, and hit the key or mike? :-)
I'm not sure if "modern" designs have overheat protection for this. I'm too old fashioned to use modern gear - I'm rather found of my old Drake, Heath, and Swan gear.
I used to work on a radar transmitter that put out a peak power of over 2MW (2 million watts). The power transformer put out 50KV at a half amp continuous. And the maggie would fry real fast if something went wrong with the waveguide.
Reply to
Zootal
I believe you, Zootal. I was there too.
Blowing the final transistor in R/C transmitters with no or collapsed antennas was warned against in some R/C system manuals. These were the ones provided by the manufacturers - so it was no myth. I even knew a few modelers that had to return their sets for repair for just this particular problem. Early Futabas (MRC) seemed to be the worst for losing the final transistor, though other brands did too.
Later on, the problem seemed to virtually disappear. But out of habit, I still, to this day, extend the antenna at least partially when turning the Tx on.
Ed Cregger
Reply to
Ed Cregger
I dunno what peak power is for the USA, but in this country it used to be about 500mW.
You can get a TO5/T)39 type transistor with a heatsink that will absorb ALL of that and more and not fry.
I built a 27Mhz SC transmitter years ago..pulled about 280mA off 18v of dry cells.
The output transistor got hot, but it never blew. I estimated around 600mW of heat..so the rest of the 5+W must have been going up the antenna..I used to enjoy flying my superregens in the company of superhets..
Reply to
The Natural Philosopher
Early on people used little plastic cased transistors. Possibly with marginal voltage ratings (you CAN get very high voltages on unmatched stages)
Also I think you can adjust the output coil design so that current falls on an unmatched aerial.
Then the manufacturers realized the extra cost of a more sturdy output transistor was worth the reduction in warranty claims.
Reply to
The Natural Philosopher

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