Winding toroids for end fed antenna

[Captain Kilowatt]

For some reason MFJ used an 'X' instead of 'J', no idea why. But the 'X' and 'J' are the same thing, reactance.


- 'Doc

Doc, MFJ did that because the "J" value is the reactance and the symbol for reactance is the letter "X". usually we see Xc for capacitive reactance and XL for inductive reactance. Since the MFJ meter does not show whether the reactance is C or L ( you have to determine that on your own by retuning the meter) they simply use the "X" to indicate the reactive component of the impedance.

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.

No you do NOT multiply those numbers. They are two separate components that make up the impedance. Ideally you want the X to be as near zero as you can get. Since you have approx. 66 feet of wire,which is near 1/2 wavelength on 40m, I think you need to check it out again with the analyzer. You may be seeing a resonant point down near 2.9 MHz due to a variety of factors besides just wire length. Check it out again and tune from about 6.0MHz up to 8 MHz or so and look for a sudden peak in the R value. This should occurs as the X value drops. Don;t worry if the X never gets to zero but it should be something quite low. In the single digits even. The 1/2 wave of wire should show many hundreds of ohms even at that height above ground. I can see my MFJ analyzer over-range (1500+) when looking into a 1/2 wave wire.

 

[KC9Q]

Camerart,

Just use the metric formulas: 1/4 wave = 71/F(MHz), 1/2 wave = 143/F(MHz), or 1 wave = 285/F(MHz). You may be using this, but if not: Use ferrite core material for your core(s). A permiability of 125 to 250 should work the best for your application.

73,
Mike

 

[camerart]

Camerart,

Just use the metric formulas: 1/4 wave = 71/F(MHz), 1/2 wave = 143/F(MHz), or 1 wave = 285/F(MHz). You may be using this, but if not: Use ferrite core material for your core(s). A permiability of 125 to 250 should work the best for your application.

73,
Mike

Thanks Mike,

I'm happy with Metric, imperial or even knots on string as long as I arrive at the correct length.

I'm pretty sure I have the correct length of wire plus a bit. I want to make sure we are all dealing with the correct thing. This is supposed to be a 1/2 wave end fed antenna for 40Mtrs. I'm wondering if there is some kind of mix up (perhaps at my end), as this seems to be 2 X 1/2 waves long, as if it is both sides of a 1/2 wave length of a dipole, but fed from one end.

I have the recommended Toroids, waiting for me to understand part of the exercise first.

Cheers, Camerart.

 

[camerart]

Doc, MFJ did that because the "J" value is the reactance and the symbol for reactance is the letter "X". usually we see Xc for capacitive reactance and XL for inductive reactance. Since the MFJ meter does not show whether the reactance is C or L ( you have to determine that on your own by retuning the meter) they simply use the "X" to indicate the reactive component of the impedance.





No you do NOT multiply those numbers. They are two separate components that make up the impedance. Ideally you want the X to be as near zero as you can get. Since you have approx. 66 feet of wire,which is near 1/2 wavelength on 40m, I think you need to check it out again with the analyzer. You may be seeing a resonant point down near 2.9 MHz due to a variety of factors besides just wire length. Check it out again and tune from about 6.0MHz up to 8 MHz or so and look for a sudden peak in the R value. This should occurs as the X value drops. Don;t worry if the X never gets to zero but it should be something quite low. In the single digits even. The 1/2 wave of wire should show many hundreds of ohms even at that height above ground. I can see my MFJ analyzer over-range (1500+) when looking into a 1/2 wave wire.

Thanks Captain Kilowatt,

I tried Analysing between 5 and 10MHz. I couldn't get a positive reading. X was between 500 and 102 and R was between 200 and 50.

The only time I could get anything that looked to me like a proper reading was around 3Mhz, when it did look like you explained. (Please read Post #18 to make sure we are on the same wavelength.)

If the readings are subtle, I can make a graph for you.

Cheers, Camerart.

 

[camerart]

Hi,

I'm still trying to get clear results, but haven't succeeded so far even with all of you help.

Someone told me, that the analyser doesn't like simple wires because it is set for 50 Ohm and the end of a wire is much higher than this, and this is why my readings are all over the place.

So if this is true then the calculation of the wire, in this case appr 66ft, is all I need to do, then work out how to calculate the toroid bit.

Z0o0o

 

[Captain Kilowatt]

Whoever told you that is wrong. The analyzer is indeed set to reference 50 ohms but that is what it uses to calculate the SWR which is referenced to 50 ohms. BTW there is a setting on the analyzer where you can change that 50 ohm reference to work in other antenna/feedline impedance systems. To be truthful I am at a loss to understand what the issue is. I have used the MFJ-269 on lots of wire antennas including an inverted L type where I wanted to know the feedpoint impedance so I could install an impedance transformer to raise the feedpoint impedance to something more suited to 50 ohm coax. I got lucky as the impedance was around 10 ohms and a simple 4:1 made a much better match to the cable. 66 feet of wire should show a very high impedance around 7,14, 21 and 28 MHz. It should show a MUCH lower impedance around 3.5,10.5, 17.5, 24.5 MHz etc.

 

[camerart]

Whoever told you that is wrong.

I'm getting advice from all directions, it's difficult to know which is good and which is bad

The analyser I'm using has been loaned to me, and I'm a bit reluctant to experiment with it too much, until I know what I'm doing

Can you explain how to change the BTW setting please?

Cheers, Camerart.

 

[Captain Kilowatt]

Although you CAN change the reference impedance the meter uses it will do you no good to do so in this case since you are working with a 50 ohm source. The need to change it comes when working with 75 ohm systems for example as used in TV or FM radio systems or when using it with odd impedance cables like 98 ohm stuff (rare).

 

[camerart]

Although you CAN change the reference impedance the meter uses it will do you no good to do so in this case since you are working with a 50 ohm source. The need to change it comes when working with 75 ohm systems for example as used in TV or FM radio systems or when using it with odd impedance cables like 98 ohm stuff (rare).

If I understand what you are saying, I can't use the analyser for tuning a wire? Is this correct?

 

[HomerBB]

If I understand the dialogue here, what is being said is that you can determine the resonant length of the wire regardless of whether you've gotten an SWR match.

I believe this is done in the Advanced Mode on the 259B, in the Resonance Mode.
In that mode resonance is read as X=[0]. Regardless of the SWR reading the wire should read for resonance at the frequency it is of proper length, that is, by working through the frequency adjustment knob your will find where the wire is currently resonant, and you can adjust the length to move resonance to where you want it to be.

When you have found resonance you will need a matching network of some sort to obtain a 50 Ohm match for your end fed wire bringing your SWR into line.

If my understanding needs correction someone will straighten it out.

 

[Captain Kilowatt]

If I understand what you are saying, I can't use the analyser for tuning a wire? Is this correct?

No. Completely the opposite. You can use the analyzer for tuning any antenna or feedline.

If I understand the dialogue here, what is being said is that you can determine the resonant length of the wire regardless of whether you've gotten an SWR match.

I believe this is done in the Advanced Mode on the 259B, in the Resonance Mode.
In that mode resonance is read as X=[0]. Regardless of the SWR reading the wire should read for resonance at the frequency it is of proper length, that is, by working through the frequency adjustment knob your will find where the wire is currently resonant, and you can adjust the length to move resonance to where you want it to be.

When you have found resonance you will need a matching network of some sort to obtain a 50 Ohm match for your end fed wire bringing your SWR into line.

If my understanding needs correction someone will straighten it out.

Homer is correct. One thing that needs to be understood is that resonance and perfect SWR do not necessarily coincide. Resonance is simply the point at which the reactance (X value) is zero. This may occur when the R value is anything. I have had resonant antenna that presented an impedance of 8 ohms (SWR= 6:1) and I have had resonant antenna that had an impedance of more than 800 ohms (SWR=16:1).

Take the wire antenna and stick the end of the wire into the analyzer and measure the total impedance. Trim it until the reactance is zero on your target frequency and then note the value of R. Construct an impedance transformer of such a ratio that the feedpoint impedance is transformed to something near 50 ohms for direct connection to your coax cable. Simple.

The reason I said to stick the end of the antenna wire directly into the analyzer is because any coax cable between it and the analyzer can and will transform the impedance and will cause a false reading of the true impedance as read by the analyzer.

 

[camerart]

If I understand the dialogue here, what is being said is that you can determine the resonant length of the wire regardless of whether you've gotten an SWR match.

I believe this is done in the Advanced Mode on the 259B, in the Resonance Mode.
In that mode resonance is read as X=[0]. Regardless of the SWR reading the wire should read for resonance at the frequency it is of proper length, that is, by working through the frequency adjustment knob your will find where the wire is currently resonant, and you can adjust the length to move resonance to where you want it to be.

When you have found resonance you will need a matching network of some sort to obtain a 50 Ohm match for your end fed wire bringing your SWR into line.

If my understanding needs correction someone will straighten it out.

Yes, quite correct, thanks

Camerart.

 

[camerart]

No. Completely the opposite. You can use the analyzer for tuning any antenna or feedline.

Homer is correct. One thing that needs to be understood is that resonance and perfect SWR do not necessarily coincide. Resonance is simply the point at which the reactance (X value) is zero. This may occur when the R value is anything. I have had resonant antenna that presented an impedance of 8 ohms (SWR= 6:1) and I have had resonant antenna that had an impedance of more than 800 ohms (SWR=16:1).

Take the wire antenna and stick the end of the wire into the analyzer and measure the total impedance. Trim it until the reactance is zero on your target frequency and then note the value of R. Construct an impedance transformer of such a ratio that the feedpoint impedance is transformed to something near 50 ohms for direct connection to your coax cable. Simple.

The reason I said to stick the end of the antenna wire directly into the analyzer is because any coax cable between it and the analyzer can and will transform the impedance and will cause a false reading of the true impedance as read by the analyzer.

Good. I removed the coax and stuck the wire into the analyser (No earth connection) indeed I'm now able to tune x = 0, frequency 5.45Mhz (In advanced Resonance mode). Although the zero flickers 0 -50ish.

I shortened the wire (tying it back up the wire) aiming for desired frequency of 7.03Mhz, but after shortening it 10ft, the x=0 frequency was at 5.79Mhz.

Cheers, Camerart.

 

[Captain Kilowatt]

Good. I removed the coax and stuck the wire into the analyser (No earth connection) indeed I'm now able to tune x = 0, frequency 5.45Mhz (In advanced Resonance mode). Although the zero flickers 0 -50ish.

I shortened the wire (tying it back up the wire) aiming for desired frequency of 7.03Mhz, but after shortening it 10ft, the x=0 frequency was at 5.79Mhz.

Cheers, Camerart.

At this point I have to say that either something is not as it is being presented or that there is a problem with the analyzer. There is no way 20.1m of wire should show resonance at 5.79 MHz ESPECIALLY after shortening it a further 10 feet. That would make it about 56 feet long which should be a half wavelength at about 8.4 MHz. I suggest you either consult someone local that is knowledge in antenna systems or even post pictures of what you are dealing with because something is just not right.

 

[camerart]

At this point I have to say that either something is not as it is being presented or that there is a problem with the analyzer. There is no way 20.1m of wire should show resonance at 5.79 MHz ESPECIALLY after shortening it a further 10 feet. That would make it about 56 feet long which should be a half wavelength at about 8.4 MHz. I suggest you either consult someone local that is knowledge in antenna systems or even post pictures of what you are dealing with because something is just not right.

Ok. I have borrowed the analyser, and will be seeing the owner tomorrow night.

The only thing that I can think of that might be questionable, is: The far end of the wire is terminated by a porcelain insulator. The near end comes through the top of a window and is trapped by it, leaving a mtr or so, inside the room where I connect it to the analyser.

Cheers, Camerart.