Base loaded Rx

[Antonsoarer]

I fly in the UK on 35mhz. I have a few GWS rx that I use in my small slope models. Don't like having the aerial hanging out so always bury it in the wing or fuz., this can cause reception problems at a distance. I don't want to buy a small base loaded aerial but will have a go at DIY base loading to help reception. Any electronics gurus know what value choke I should use for base loading to make up for the shorter effective length? I have had some success just winding part of the aerial wire round a small former.

Tony.

[electroboy]

Though this may not answer your question directly, this link was very interesting to me and may help guide you in your quest.
Rx antenna article (http://www.rc-cam.com/ant_exp.htm)
The section of the article regarding "coiling" antenna lead and reduction of range was frightening as I was prepared to approach that method.

I too, seek the smaller rx antenna for a micro-outdoor flyer but want good signal resolution. The search continues.

toodles,
K

[Ironsides]

Mr RC-Cam:

http://www.rc-cam.com/ant_exp.htm

That is an excellent article.

David

[electroboy]

Mr RC-Cam:

http://www.rc-cam.com/ant_exp.htm

That is an excellent article.

David

See,
Mr. Ironsides agrees with me.

Mr. RC-Cam, please post your findings.

[vintage1]

Most receivers won't show a huge increase in gain with a bottom loaded aerial. Most can be tuned up a bit anyway to compensate for a shorter aerial.

The real answer is a decent reciever with a bit more gain. My Jetis all seem to work up to about 500 yards on the fairly minimal bits of wire that the come with. IIRC the small GWS has no input tuned circuit anyway - the aerial plugs straight into the mixer.

But FWIW I think a choke in the 3.3uH range is a place to start. You never know. You might get a bit of input tuning that will actually improve the GWS (lack of ) front end.

[Antonsoarer]

Thanks to everyone for the input, time to experiment. :) Tony.

[vintage1]

Though this may not answer your question directly, this link was very interesting to me and may help guide you in your quest.
Rx antenna article (http://www.rc-cam.com/ant_exp.htm)
The section of the article regarding "coiling" antenna lead and reduction of range was frightening as I was prepared to approach that method.

I too, seek the smaller rx antenna for a micro-outdoor flyer but want good signal resolution. The search continues.

toodles,
K

This article is fairly consistent with my 'gut feel' on having both messed around with a few receivers, and also messed around with brodcast receiver design in an earlier life.


That is, shortening the aerial on most sets does not lead to a disastrous loss in performance, though it does reduce range, and there is no magic 'correctly tuned' length, rather a broad range of lengths which work fairly well, and a smooth dimunition of range if that length is drastically changed from optimum.

The issue of matching/tuning/size is an interrelated one. A well resonant aerial does increase its effective size, but not by nearly as much as extra length does. Correct matching generally reduces any tuning effect anyway. And no one really bothers - as long as the range is 'good enough' then that is where the designer tends to stop.

What DOES make a HUGE difference is not running the aerial all in one plane. If you do have the aerial all in one plane, there is a null zone in the polar diagram when the aerial is 'end on' to the transmitter, and, if you also as most people do, tend to point the TX aerial more or less at the model, you can have a nasty situation when the model is - for e.g. antenna streched to fin top - coming straight towards you.

I think on balance, that the best way to fit aerials to msall models, is to buld in a wire going from wingtip to wingtip, and another gong down to the tail, and a vertical section or two bundled up inside the fuselage, and connnect them all together. You might find alse that if you can pick off the signal monitoring, as described in te article, or the audio at pin 9 on the same class of chips - or even just retreat away from the transmitter, you can tune up the input coil on such receivers as have it, and gain 5-8dB by matching everything up.

On receivers without such tuning, an external RF stage with tuning coil may be an option, if you can find tunable coils of suitable resonant frequency.

A 20-40dB gain stage will imprive the performance of any cheap receiver - very few receivers are anywhere near the theoretical noise floor of modern RF devices. Ther is a lot more sensitivity to be had.

I oightto get oine of my spare Jetis and experiment...but by and large they seem to have adequate range (300m) even on very short aerials, for my tiniest models.

[JimCob]

Here is how I have created shortened antennas for my models on the 50 MHz band. They have all worked very well and you can make them as long or as short as you want (within reason) and for any band you want.

1. This website has an on-line calculator that will give you the value of the inductor needed based on your inputs as to the overall length of the antenna, frequency, coil position, etc. Start out with some idea as to the overall length you want and go from there.

http://eweb.chemeng.ed.ac.uk/jack/radio/software/loading.html

2. Once you have your design, go to your favorite parts supplier (like digikey.com or mouser.com) and find the right kind of inductor. You're looking for a small molded choke with a self-resonant frequency above your intended frequency of use. For example, for the 50 MHz band I choose chokes with a self-resonant frequency at 70 MHz or so. If you can't find what you want change your design a little bit by moving the coil either closer or further away from your feed point. My 50 MHz designs end up with chokes in the 2.7 - 3.3 uH range. (You can also make a small air-wound coil yourself)

3. Tune it. The calculator gets you into the ballpark, but I always tune it for resonance using a antenna analyzer:

http://www.mfjenterprises.com/products.php?prodid=MFJ-259B

Do you _have_ to tune it to make it work? Probably not, but if you have an analyzer or know someone who does I think it's a worthwhile process.

Here is how I tune it up:
a) I take a magnetic antenna mount (the kind you'd use for the 2m ham band for instance), remove the whip, and mount one end of the shortened antenna in it. For this purposes of tuning I temporarily use stiffer wire for the antenna legs so the whole thing will stand up straight. I usually start with the top wire a little long so I can trim it to the exact frequency I want.

b) Plop it onto the hood of the wife's car (because she parks it right next to my work bench)

c) Connect the analyzer and trim the upper element for the best match (i.e. lowest SWR). You'll get a 50 ohm impedance because of the ground plane provided by the car. If you were to do this operation by holding it in mid-air your impedance will be something different and you'll never know when you're in resonance.

d) Remove the temporary wires and solder on whatever kind of wire you want in the finished product, like 28 ga magnet wire, and attach it to your Rx. Make sure the wire lengths are the same as what you ended up with after the tuning process.

Done!

[electroboy]

Here is how I have created shortened antennas for my models on the 50 MHz band.



Wow,
really neat stuff. My calculations indicate @ 72.650Mhz(US Ch43), a 9" antenna of 24g w/3" feed point = 3.08 mH

I dunno how I'm gonna tune it w my unique degree of ignorance and lack of proper equipment, tho'.
But I'll prolly perform some range experiments and see how it goes.

Thanks for the link.
I am interested in squeezing an appropriate antenna in one of these:
17-3/4" wingspan, 15-1/2" long,

[Bthree]

I oightto get oine of my spare Jetis and experiment...but by and large they seem to have adequate range (300m) even on very short aerials, for my tiniest models.

Had a physical look at a Jeti Rex5 (36Mhz) which has a 500mm antenna length and on the PCB is a 6.8uH choke in the antenna lead. Using the link above http://eweb.chemeng.ed.ac.uk/jack/radio/software/loading.html this gives a choke value of 4.5uH. Hmmm that didn't quite work !

[vintage1]

Near enough. Remember there are other factors at work. Not sure my REX 4's have a choke tho.

[FenceMagnet]

"But I'll prolly perform some range experiments and see how it goes."

ACK !!!!! "prolly" again !!!!

:D

[electroboy]

"But I'll prolly perform some range experiments and see how it goes."

ACK !!!!! "prolly" again !!!!

:D

is there a prollem?


just economizing the typing. No real threat to the english language, I deem.

[FenceMagnet]

"is there a prollem?"

:)

[Antonsoarer]

Electroboy, great info in that article, Bthree, good calculator too. Vintage, I will also try the multi-plane thing because that's how I get glitches, edge on at a distance with aerial across the wing. I will get some inductors and start testing.

Tony.

[grumman5277]

I have a friend that works on equipment in this very area. He mostly deals with
military applications and various affects on equipment within frequencies the
aircrafts equipment may encounter during flight. He asked what frequency I
was using for my planes and he came up with the length of the antenna within
two inches. Thats not bad for off the top of your head . I send this his way
and see if he can shead some light on it.

[C Andrew Green]

Tony,

Only just came across this thread, but here's a Link (http://www.iroquois.free-online.co.uk/aerial.htm) to a design for 35Mhz using parts available in the UK from Maplins (part numbers in the article) same parts are available from Farnell, I've got some chokes just got to build one and give it a try.

Andrew

[grumman5277]

Here is some new info that I recieved from a local HAM guy.

Take 468 divided by 72MHZ or 468 divided by 72 million
That will give you the length of wire needed to make the (choke) coil.
You will need to test to make sure you are getting the proper range though.
The wire needed for the choke is very thin magnet wire 30 and higher gage
to keep it light.