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RX Ant. Length

Old 12-29-2003, 01:41 AM
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PaulVi
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Default RX Ant. Length

On a small plane with lets say a 20" Span. what can you do with the extra ant. wire that hangs off the back. I know that it works fine and have seen replacment ant. (range reduced) but is there anything we can do with our extra length that will not dramaticly reduce the performace (distance) of our recievers.

What comes to mind is a mid load like a CB radio would use. what would the air core have to be Soda Straw? how many wraps would we need to use? is there a simple caculator that could be used IE plug in the freq. and the core diamiter to get back the number of turns and the un coiled length.

I know that the ones on the market are $10 but it would be nice from a hobbie point of view if we could wind our own.

Any thoughts?
Old 12-29-2003, 10:59 AM
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Mike Taylor
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Default RE: RX Ant. Length

Some small Rxs come with a 1" wide card that allows you to wrap up as much of the excess as you need to. I seems to shorten range only a little bit, and it does neaten up the model's appearance not to have a cable flying in the breeze. You can also wrap your existing antenna around a soda straw; just don't cross any wraps over the others.

There are also base-loaded antennas like the Azarr's or Jim's Litetenna. These are about 5" total length, but they do shorten range 15%.
Old 12-30-2003, 12:37 AM
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PaulVi
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Default RE: RX Ant. Length

Thank you for your response. I guess I was hopping for a bit more technical data. I am sure there is a formula the depending on core material wire gadge and such as well as frequency you are trying to tune. That one could wrap your own. I know that the electrical length changes on a antenna when it is coiled. Or loaded as such. I used to build CB radio antenna back in the 70's but my books have long gone the garage sale way.
Old 12-30-2003, 06:33 PM
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kenru
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Default RE: RX Ant. Length

The formula is 2952/Freq (in MHz) = 1/4 wave antenna in inches. The wire size doesn't matter because there is immeasurable current flow in the wire. The antenna may be wound around a non-conductive form or left straight with the only change in characteristics being a slight reduction in range for the wound configuration.
Old 12-30-2003, 08:53 PM
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Default RE: RX Ant. Length

You could cut the antenna in half. But expect about 500' range. Thats fine for indside flying or park flying.
I do this for all micro rx's used for electric park flyers, works fine.
Anyone who hasent heard of this, check around before you jump on me. It is a common thing to do.
Old 12-30-2003, 11:18 PM
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PaulVi
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Default RE: RX Ant. Length

That would make sense. Effectively you would be running a 1/8 wave give or take. I decided to play a bit with a very unscientific method I will pace off 300'. start with the stock ant. and then try the 1/8 wave idea. (Still will not be as clean as I want but better). then I and going to wind a few ant. around a soda straw. starting at 32"and working up to 45" in 1 inch increments then just go to the park with a partner and start walking away with the xmiter ant. down and look for erratic behavior. I will make a quick change plug using pins and plugs from a DB9 connector (have then in a bag) and see where I get.


If I can be consistent with my testing I will put the results up here.

On the note of Ant wire. I took a phone cord 25' silver satin ext cord and striped the plastic over coat off, inside were 4 wires 1 each black, red, green, yellow about 24-26 Gage so I have an ample supply or replacment wire that cost nothing.

We will see.

If anybody has any toughs please jump in. if we can do some realistic testing. we may be able to save a few bucks in an other wise expensive hobby..
Old 12-31-2003, 01:01 AM
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PaulVi
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Default RE: RX Ant. Length

1.5" is from silver satin and the 1" is wire wrap wire. the 1" is solid core wire and the 1.5 is stranded.

the 1.5 is 1.5 grams and the wire wrap is .8 grams I think I will try a coffee stirring straw and wire wrap wire to see if I can get it lighter.

But if what every one is saying is true they both should work well, the Stock GWS Pico is 36 and 3/4 inch my Futaba is 39inchs and if I calc for 72.130 (ch17) i get 39.9inch
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Old 12-31-2003, 01:30 AM
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RichLockyer
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Default RE: RX Ant. Length

ORIGINAL: kenru
The formula is 2952/Freq (in MHz) = 1/4 wave antenna in inches. The wire size doesn't matter because there is immeasurable current flow in the wire. The antenna may be wound around a non-conductive form or left straight with the only change in characteristics being a slight reduction in range for the wound configuration.
This is interesting. I've often wondered about base/mid/top load antennae and how the length is calculated.
So on the common "big stick" CB antennae, are those actually 1/2-wave loads coiled onto the 48" stick?
If so, then we should be able to double the length of our RX whips, then coil them into a compact whip, and get performance pretty close to stock.
Old 12-31-2003, 01:44 AM
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Default RE: RX Ant. Length

That is kinda where I was hoping to go with this. Continuous loaded ant's tend to have poor impedance matching so it is tough to get great performance. Keep in mind the GWS naro's have a 500' range real world it is less due to noise. but if we can compact the ant. and still keep the original reception strength Ya Hoo to us.

[link=http://viscovich.dhs.org/calc.htm]Wave Lenght Freq Calc[/link]

I have posted a calculator and freq chart so you can get an Idea what WL 1/2WL and 1/4WL's look like
Old 12-31-2003, 04:49 PM
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Default RE: RX Ant. Length

That 500' range is with a 20" antena. And it is a good 500 plus feet. I fly in LA where we have very active air waves, and have no trouble.
It seems that coiling the antena wire would only cancel its own signal unless you use a centain circuit.
I'm interested in your test resaults.
Old 12-31-2003, 05:36 PM
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Default RE: RX Ant. Length

Looping the wire, as if wrapping it around a tire, with the start and endpoints on the same plane (imagine laying the wire flat, then rolling it up on the wheel) would indeed cancel.
What we're talking about is a loading coil, like in a coil spring, where the endpoints are along the axis of the coil, and none of the wraps actually cross each other.

Yes, there is some signal loss compared to a straight 1/4 or 1/2 wave whip, but the load and SWR would still be correct.
Old 01-14-2004, 02:09 PM
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Default RE: RX Ant. Length

I have not forgoten this thread. I have been in the building pahse of 2 new planes as soon as they are done I am going to start testing the ant. ideas out..

build 1st fly 2nd (No mater how many times I tosses the box in the air it would not get airborne...)
Old 01-15-2004, 06:24 AM
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Default RE: RX Ant. Length

Or, you could always use one of these.

Azarr
www.ecubedrc.com
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Old 01-16-2004, 02:44 AM
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Default RE: RX Ant. Length

From what I have read your Ant. is one of the best reduced length Ant. out there. I would not want to take anything away from your design. I (we) were just looking for options. The last ant. i bought ($10) came apart when I was striping a plane (Not Your Ant.) so I got to thinking about what it would take to make a reduced length Ant. that could be disposable.

I used to have a few books on Ant. design way back when (CB DAYS) but have long since misplaced them. So I started looking on the net for info (found almost none) So the next best thing being a hobbist is to experiment and see what we could do.

Tuneing a reciving Ant. is not as critical as a xmiter antena because you do not have reflected power so nothing to burn up. But getting the inductance correct and getting a electrical 1/4 or 1/2 wave is much harder than it looks.

You can not just take a 1/2 wave length of wire and wrap it around a arbitrary sized core. (Soda Straw) inductance will change the electical length of your wire and it may need to be longer or shorter than the wave length that you are building for.

So the idea was to stir some intrest to see if we could maybe come up with a working reduced size Ant. but more importantly a formula or two to aid in the design.
Old 01-16-2004, 10:18 AM
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Default RE: RX Ant. Length

I'm new here but after reading this thread, maybe I can be of some help. I use the GWS 4 receivers and for several years have made my own "Dean's Style " antennas. I have , of course only by accident, ocassionally had my planes so far out I had trouble seeing their direction. Never once did I loose radio range.

I have attached a photo on one of my planes. The antenna is hard to see but you can see the antenna wire from the receiver, the small circuit board and if you look really close you can see the wire mast with a little loop at the top.

This is a modification of Tony Van Roon's "Fly Wire" antenna

http://www.meridianelectronics.ca/gadgets/flywire.htm

It works really well with the GWS 4 receivers but I haven't tried it with others. I leave 6" of wire antenna from the receiver, a 1.5 microhenry choke and a 6" wire mast with a little loop on the end so you don't get stuck.

Jerry Tuttle[img][/img]
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