Winding toroids for end fed antenna

[camerart]

Hi all,

I am trying to understand antennas a little more, I realise there are many variables, but I simply want more the rule of thumb information and nothing too complex.

I have an antenna analyser, Oscilloscope, signals generator, L/C meter and of course a standard electrical meter.

I have been experimenting with winding toroids for end fed wires and have some information, but am having difficulty putting it into a method of working.

I think I first need to find the impedance of the wire (20.1 Mtrs long for 40 Mtrs), then calculate the wire winding ratio, then match the wire with a capacitor. I have succeeded and failed after a number of attempts, so it isn't consistent.

If this is correct, first can someone tell me how to get the wire impedance form the long wire, please?

Cheers, Camerart.

 

[W5LZ]

I may be misunderstanding what you want, but, I would think that with that antenna analyzer you could just measure the wire's input impedance. That would at least give you a starting point.
An end fed wire of about a 1/2 wave length will have a higher than 50 ohm input impedance, that's fairly 'standard'. It can range from several hundred ohms to more than a 1000 ohms depending on height, etc, so measuring your antenna would be 'closer' than just guessing.
That analyzer should give you both a "R +/- J" reading and an SWR reading, right? So depending on which you would find more useful, there's your starting point for impedance. Then it's a matter of determining the required '+/-J' required for that torroid assembly.
Is that easy/simple? Not really, but it's about as 'simple' as it get's.
Any help?
- 'Doc

 

[camerart]

Hi W5LZ,

Yes, this is what I am looking for.

I have a 20.1Mtr long wire approx 4 Mtrs from the ground. I tuned the Analyser to 7.030Mhz for a fixed 40 Mtr frequency radio. It reads R=24-30 (this kept changing) X=316 and for SWR it showed >25 (This I assume to be out of it's range)

Thanks Camerart.

 

[RADIOOMAN]

Hi W5LZ,

Yes, this is what I am looking for.

I have a 20.1Mtr long wire approx 4 Mtrs from the ground. I tuned the Analyser to 7.030Mhz for a fixed 40 Mtr frequency radio. It reads R=24-30 (this kept changing) X=316 and for SWR it showed >25 (This I assume to be out of it's range)

Thanks Camerart.

So you are wanting to end-feed a half-wave wire.
I am not sure why your "R" value is so low.
The "R" value for a end-fed 1/2 wave should be quite high.
Maybe I should know more about the configuration of the wire.
Is your antenna wire vertical or horizontal?
You might consider the zep approach, you could get a variable "C" and a fixed "L" of the appropriate value and configure for a parrallel resonant circuit or you could use a antenna tuner.
By using the antenna tuner that will allow you to utilize this wire on other frequencies if needed.

 

[camerart]

Hi Radiooman,

I am able to make a Toroid plus capacitor all tuned correctly, but what I'm trying to do, is to better understand how it works.

The wire is 4Mtrs from the ground (Both ends). I don't understand the analyser reading either. Perhaps it can't analyse a simple wire.

I want to understand the wire first, and make sure it is the exact correct length and quality, then work out the toroid windings, and capacitor needed to make a tuned aerial.

Cheers, Camerart.

 

[RADIOOMAN]

Hi Radiooman,

I am able to make a Toroid plus capacitor all tuned correctly, but what I'm trying to do, is to better understand how it works.

The wire is 4Mtrs from the ground (Both ends). I don't understand the analyser reading either. Perhaps it can't analyse a simple wire.

I want to understand the wire first, and make sure it is the exact correct length and quality, then work out the toroid windings, and capacitor needed to make a tuned aerial.

Cheers, Camerart.

If you are saying that the wire is horizontal then that seems to be consistent with the readings from the analyzer.
A horizontal wire that long and that close to the ground will have a low "R" value to its impedance.
The analyzer can give you the correct readings from the wire.
The analyzer is just like a lo-power transmitter with swr meter but additionally it has the ability to indicate the resistive part and the reactive part of the complex impedance.
The information you are looking for is in multiple places on the internet or in radio handbooks.
And, additionally, the books employ graphs and charts which help the reader to comprehend the subject matter.
I am not saying that there are not people in this forum that could not post this information for you to read, but to encompass this subject reasonably well it will take a lot of typing and that seems to me to be an inefficient and less effective method ( and somewhat unfair ) for you to utilize for the purpose of comprehending the concepts involved with this subject since the information is readily available in multiple places elsewhere.
There are any number of people in this forum that can parrot this information to you but they most likely would have gotten their familiarity of the subject from reading it in a book or from the internet.
I don't want to seem like a person that comes across as the guy that thinks " I got it from a lot of reading and think and you should do it the same way"
One of the reason why I think it would be better for you to use my recommended approach is because there are going to be some people that will share their opinion on the subject but they themselves don't really understand it and just use a bunch of words to try and make you think they know what they are talking about...... I am wondering now if I should just stop talking......lol
Anyway _... _ .._ _._

 

[camerart]

Thanks Radiooman,

I have read a lot about it, but I'm sure there is a way, that a wire can be analysed, and fairly easy rule of thumb methods can be used to work out the toroid and capacitor, without lots of explanations.

Cheers, Camerart.

 

[RADIOOMAN]

Thanks Radiooman,

I have read a lot about it, but I'm sure there is a way, that a wire can be analysed, and fairly easy rule of thumb methods can be used to work out the toroid and capacitor, without lots of explanations.

Cheers, Camerart.

I am not sure if that is your way of dismissing me from further discussion on this topic, but if it is, that's ok with me.
If it is horizontal and you had read any books about antenna wires you would already know that your wire would exhibit those kinds readings on an antenna analyzer.
And there are any number of places on the internet that will provide you with a mechanism that will allow you to plug in your impedance values and crunch the numbers and then show you how many turns and what value of inductance and what value of capacitance needed to transform it to the desired "R" and "X" values.

 

[W5LZ]

The 'R+/-J' you are seeing, mainly the 'J' part, is telling you that the antenna is not resonant on the frequency you expect. Twiddle that freq. knob till the 'J' value is as low as you can possibly get it. That's where the thing is resonant. The 'R' value has nothing to do with resonance, it'll end up some value where that 'J' is at it's lowest (hopefully zero), and that's basically what the antennas input impedance will show. The SWR meter also can't tell you anything about resonance. A low SWR doesn't indicate resonance at all, only that the complex impedance, meaning the combination of both the 'R' and 'J' are the -equivalent- of 50 ohms. A 'J' number, the reactance, or zero means resonance, that's by definition.
'RADIOOMAN' is right, the antenna's height above ground can change the antenna's input impedance. The higher it is the less effect the presence of ground/dirt will make. But that's just one of the things that can affect input impedance, there are others.
Something I didn't see is, what are you using for a 'ground', or counterpoise, or the other half of the antenna? There has to be one. If 'all' of the antenna is at the same height then I have to assume you are using coax to feed it? So is the braid/ground of that coax connected to anything?
- 'Doc

 

[camerart]

I am not sure if that is your way of dismissing me from further discussion on this topic, but if it is, that's ok with me.

Not at all radio man, your input is welcome, but saying I've read a lot about it, doesn't mean I remember it. If I play with actual wires and readings, I more get the feel of what's going on.

Cheers, Camerart.

 

[camerart]

Can I break it down a little and do it in stages, please?

Twiddle that freq. knob till the 'J' value is as low as you can possibly get it. That's where the thing is resonant.
- 'Doc

I twiddled = Result @ 2.913Mhz--SWR1.1-- x 0--R 58

I'm trying to understand 'j' Is it the same as 'X' ?

I assume the wire is resonant @2.913Mhz so do I cut it or lengthen it to make resonate @ 7.030 Mhz

Something I didn't see is, what are you using for a 'ground', or counterpoise, or the other half of the antenna?
- 'Doc

I have an earth wire from the central heating radiator.

If 'all' of the antenna is at the same height then I have to assume you are using coax to feed it? So is the braid/ground of that coax connected to anything?
- 'Doc

Nothing except the Analyser.

Regarding height above ground. This is an experiment, to make a fixed antenna for a fixed frequency radio. I imagine I would throw a string up a tree with one end of the antenna tied to it, and the bottom (radio) end staked to the ground, in the correct way.

Cheers, Camerart.

 

[W5LZ]

...well, had a beautiful explanation all typed out and 'lost' it. I seem to be doing that a lot lately. I'll try it again...

'Take it in stages', good, that works.

That 'X' and 'J' thingy is a 'mystery'. If you study the theory reactance is expressed as a 'J', (R +/- J). For some reason MFJ used an 'X' instead of 'J', no idea why. But the 'X' and 'J' are the same thing, reactance.

"I twiddled = Result @ 2.913Mhz--SWR1.1-- x 0--R 58"

And considering the length, that indicates that the analyzer 'thinks' it's looking at a 1/4 wave antenna, not a 1/2 wave. It also indicates that the coax feed line is acting as a part of the over all length and giving the results you got (58R+/-0X) at 2.913 Mhz, which works out to a 1/4 wave length at 2.913 Mhz approximately. If that coax wasn't acting as part of the antenna's length then that 1/2 wave at 7.030 Mhz would look like a resonant 1/4 wave at 3.515 Mhz. There's too much difference there (7.030/2.913) for just the reactance present in the coax to be making the difference.
So, as it sits, if you shortened that antenna it would then become resonant at the desired frequency.
Before you run out and start shortening the thing, there's another part that has to be considered. Is that ground to the heating radiator connected to the coax when you are using the analyzer, or is it connected to the radio instead. If it's only connected to the radio and not the coax and analyzer, then it's not part of the antenna system when you are using the analyzer. So the readings you are getting aren't going to be the same when the coax is connected to the radio. Make that connection and take the readings again. (Hopefully I'm wrong in that ground connection guess but better make sure, you know?)
Quick-n-dirty antenna 'scaling'.
Multiply the length times the resonant frequency then divide by the desired frequency. That will give you an approximate length. Add a foot or two to that for a fudge factor so that you don't have to add any length to it if you cut too much off. Then you get to tune it, make adjustments to the length till things are 'right'. (Hint: Don't cut anything off, just fold it back onto it's self. It amounts to the same thing and is easier than adding some on later.)
If you have to use an end fed wire antenna then do so. But, center feeding a 1/2 wave antenna is much easier, if you can do so conveniently, got the room to string the @#$ thing up. The input impedance of an end fed 1/2 wave is going to be high. The impedance of a center fed 1/2 wave is generally much closer to 50 ohms. There are two things you always have to do with any antenna. They are make them resonant and matching impedance to the rest of the antenna system. How you go about those two things is certainly not limited to just 'one way', there are several ways. Each of those 'ways' have their own benefits and problems. Do it the best/easiest way you can.
I hope some of that made sense...
- 'Doc

 

[camerart]

Hi W5LY,

I've done the 'type out a considered reply, then lost it, a bit annoying! I have started to copy and sometimes paste onto a TXT document, until it's safely gone, but not always

Thanks for your explanation.

The earth wire: Yes, the earth wire is connected to the braid of the coax. The 20.1+MTr wire is connected to the centre.

I have changed the coax to a known 50ohm 1Mtr length and taken the readings again. Similar to before: SWR= 1.1--J= 0--R= 58. Resonant frequency 2.913Mhz.

If I understand your calculation correctly: Length 20.1Mtr X 2,913Mhz =58551/Desired frequency 7.030 =8.3 Mtr wire.

This seems too short to me. I was told somewhere near 20.1Mtrs for 1/2 wave end fed antenna. Perhaps it's the end fed bit that makes it confusing?

This first project is for an end fed antenna to suit a FOXX-3 fixed frequency radio, for a fun little unit. After that I intend to make an ATU, hoping that I understand setting up a little better.

Cheer, Camerart.

 

[W5LZ]

Camerart,
The confusing part for me is the metric system! Manipulating the 'numbers' is easy, but those lengths just don't 'mean' any thing to me, you know?
So, in feet here's what the lengths would be.
A 1/4 wave is found with '234' divided by the frq in Mhz, so;
234 / 7.013 = 33.36 feet

For HF antennas that length is always going to be very slightly long, so figuring to a single decimal place usually works fine. It's a 'ball-park' number and will always be subject to some 'fine' tuning to get it as close as you want. So, converting it to meters it'd be '10.15' meters... right? close?

Making your antenna about 10.25 or 10.33 meters long then checking for resonance with you analyzer should be close to the frequency you want, a 1/4 wave antenna. IF you want to deal with a 1/2 wave length just double that 10.15m to 20.3m. You won't like the resulting impedance mismatch, but that's what finding out how to make an impedance transformer thingy was about in the original post, right?
Any 'drastic' mistakes I've made are because of that metric conversion thingy! That's my excuse anyway!
Make any sense now?
- 'Doc

(I'm trying to do all this stuff one-handed and in the car. So, that's another excuse I'm using for any stupid mistakes. Oh, and I'm out of coffee! That's the biggy!)

 

[camerart]

Thanks W5LZ,

Yes, the metric system is a bit difficult at first. We in England changed to metric many years ago, and have to work in both systems when dealing with different countries. We also converted money from imperial to decimal £ S D Pound, shillings and pence. 12 pence in a shilling, 20 shillings in a pound.

Anyway back to Antennas (or do I mean Aerials)

My Antenna is 20.1 (66Ft) (1/2 wave) As we have discussed this is resonant at 2,913Mhz.

Can you give me the calculation again please, using Yards, Feet and inches is ok. To tell me how long the wire should be, to resonate.

I am assuming that once it is resonating at 7.030Mhz, I multiply j and R together to find the impedance. I'm hoping for somewhere around 4-5000Ohms.

Cheers, Camerart. Hope you got your coffee ok.