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(ASK) How to measuring real impedance of halfwave antenna's element?

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bubun stupid

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i read an antenna book that say impedance at end point of halfwave antenna variously in range 2000Ω to 5000Ω. how i can measuring it in practice? i have vswr meter, 50Ω, but the scale can not show 1:40 - 1:100. please anyone advise me. thankyou.
 
Thanks alot JimB..!
you give me best. after i read i think i must use Antenna analyzer like as MFJ sell, does it? unfortunately it is high price for me.
if only i can design and make a real known value impedance transformer with some coil by myself, it will much easier.

Bubun Stupid
 
Hi, Nigel.
i want to kow real impedance at end point of various dipole element with various diameter i made . so i hope can calculating a Q-match or matching coil better.
 
What frequency are you using?
JimB
 
at VHF region. I make for 70MHz, 4 m band. it's 'empty' frequency in my place. so i often use for experimenting antenna.
Thank's alot JimB
 
It has been 40 years so my memory is cold....
I made a Wheatstone bridge. Replace the DC supply with a 70mhz AC source. Use high frequency parts.
I think my antennas were in the 20 to 200 ohm range. I think it worked up to 2k ohm. I never tired above that.
 
Thanks ronsimpson. your idea like as me and i had tried that. i made wheatstonebridge from 50 ohm resistor. but it was hard to marking more than 1:5 VSWR point. it just straight to infinite. may be you have some tips for overcome it?
 
If you change the 50 ohm resistor to 500 ohms it will help your readings.

I think antenna testing is covered in the ARRL Antenna Book.
 
yes ronsimpson, if only i can do that simply. but if i change resistor bridge to 500 ohm it mean i must use a paralel transmission 500 ohm from bridge to measured antenna, right? it was not easy in my place although i can make it by myself. I have ARRL Antenna Book 21st but it contents just showing antenna in 'modified' region impedance that neared to normal value. I see j-pole in this book but can't find reference to measuring real antenna impedance at end point of dipole.
Thankyou ronsimpson, i'm very appreciate your help.
 
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I see now.

You have 'transmitter' in the house, long 50 ohm wire, and antenna to test.

I test the antenna with the bridge at the antenna so the coax does not effect the readings. I think you can not get good numbers with a coax because you are test a 'system' not just the antenna. If you change the length of the coax you will get different numbers. Example; if you measure the impedance of a coax (open on the far end) of 1/4, 1/2, 3/4 or 1 wave length you will get very different numbers.
 
it's right about coax can altering true number . Seem.. it's good idea to trying put the bridge directly at antenna's terminal. I will try soon but i think i should to prevent wild rectance may occur, does it?
Thanks alot, ronsimpson.
 
I see j-pole in this book but can't find reference to measuring real antenna impedance at end point of dipole.

I have been reading through this thread and am still trying to figure out what/where you are trying to measure?
WHAT do you mean by end point ? do you really mean feed point ?

The feed point is the only place you need to be interested in the impedance and that is already a known + or - a few ohms for the 2 main styles of dipoles.
A standard 1/2 wave dipole is 75 Ohms and the folded dipole is 300 Ohms when they are at resonance at a given freq.

The 2 primary times that varies is ....
1) the dipole isnt cut to resonance at the required freq
2) the dipole is placed with other elements to form a yagi

The antenna analyser, like MFJ sells, is good for that final fine tuning of the dipole to bring it into resonance. or if its in a yagi the varying of the spacing of the reflector, driven element ( your main dipole) and the first few directors to bring the system into resonance.

Personally I have never used an antenna analyser in the 30+ years of building antennas. Like you, i have thought, for what it is, its too expensive haha

on the other hand a decent reflectometer for reading forward and reverse power is all you really need, and tune the system for max forward and minimum reflected power.

Ok granted, for what ever reason, you may actually want to know the real impedance at the feed point, and I guess, interesting from a learning and informative point of view. There's probably very few people in the real world that could quote you what the feedpoint impedance is of their antenna.
My commercial ones I use the datasheet that came with the antenna and hope the manufacturer could do a reasonably accurate measurement.
( Hey its in resonance and radiates well, so cant be too bad ;) )
my home built antennas I just test as above and tune for max forward/min ref. power

The effect of placing a dipole into a yagi system has the effect of reducing the impedance of the feedpoint. a plain dipole can drop to as low as 25 Ohms from that original 75 Ohms.
A 300 Ohm folded dipole in a yagi system can drop down to ~200 Ohms. Which is pretty handy as you can then use 50 Ohm coax and a 4 : 1 BALUN for a good match

just a few random thoughts :)
cheers
Dave
 
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I,m sorry ALL..! I just backed home from somewhere..
Hi, davenn.. I mean end point a halfwave element. I read from many book that the impedance at END POINT in range 2000 Ohm to 5000 Ohm relatively. So, i want to know the exact value of halfwave antenna's element that i made. If ever i know, i can planning a matching configuration for that like as determine matching coil or a stub. I knew that at center point (center feed) the impedance of halfwave like as you explain (near 72Ohm for 'single' and near 300 for 'folded'). I believe you can help me about it..
Thanks alot davenn..!
 
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I,m sorry ALL..! I just backed home from somewhere..
Hi, davenn.. I mean end point a halfwave element. I read from many book that the impedance at END POINT in range 2000 Ohm to 5000 Ohm relatively. So, i want to know the exact value of halfwave antenna's element that i made. If ever i know, i can planning a matching configuration for that like as determine matching coil or a stub. I knew that at center point (center feed) the impedance of halfwave like as you explain (near 72Ohm for 'single' and near 300 for 'folded'). I believe you can help me about it..
Thanks alot davenn..!

end points are never referred to when talking about impedance of an antenna, you are getting your terminology mixed up. As I said in my previous post the FEEDPOINT is the only place you need to be worried about impedance. And as I said, this is already relatively well known.

a 1/2 wave dipole length is worked out by the following formula

wavelength (metres) = 300 / Freq (MHz) so for your 70MHz .... wavelength = 300 / 70 = 4.28 metres

for a 1/2 wave dipole = 4.28 / 2 = 2.14 metres
Have a look at this basic diagram.....

dipole-feed-gif.58929


The feedpoint of this dipole will be ~ 75 Ohms give or take a couple of Ohms.
the two aluminium elements would be mounted on a plastic block. These blocks are commonly available from places that make/sell TV antennas.

Dave
 

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Now for receive only you will get away with connecting the coax cable directly to the elements as shown. Braid of the cable to one element and the core of the coax to the other element.
Strickly speaking its not a perfect way to do it. Reason is, coax cable is an unbalanced transmission line and a dipole is a balanced system.
For receive only and low power transmit it is acceptable and will work ok. For higher power transmit say 10W and up, I would use a BALUN.
a BALUN is a BALanced to UNbalanced transformer to match feedline to feedpoint.
If a balun isnt used RF currents are likely to flow down the braid of the coax and be radiated ...This can cause all sorts of problems.

If you really insist on wanting to measure the feedpoint impedance, then you will have to spenmd the money and buy an antenna analyser of some description and ensure it covers the frequency of interest to you.

cheers
Dave
 
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