EEVblog Electronics Community Forum

General => General Chat => Topic started by: kj5cn on March 15, 2012, 06:13:03 pm

Title: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: kj5cn on March 15, 2012, 06:13:03 pm
http://www.ehow.com/how_6085455_measure-uhf-antenna-resistance.html (http://www.ehow.com/how_6085455_measure-uhf-antenna-resistance.html)

Why oh why did I have to buy all those expensive RF gadgets??? :)

-Peter.
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: DonRon on March 15, 2012, 06:36:47 pm
Hi Peter,

that's great!!!

I will sell my network analyzer immediately and buy one of these cheapo multimeters (don't think it will work with my expensive DMM).

Do you think i can use it to measure the impedance of my shortwave antennas??  :-)

Cheer's

Ronald
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: kj5cn on March 15, 2012, 06:40:08 pm
Yep, a half-wave dipole should show exactly 75 Ohms. Anything else - it's broken, throw it away  :)
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: sacherjj on March 15, 2012, 06:41:46 pm
Most yagi style TV antennas are 300 ohm impedance and use a 4:1 balun to balance to 75 ohm coax.  I couldn't resist but to post something on there.  They also have a nice article on measuring speaker impedance with a multimeter.   ::)
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: Mechatrommer on March 15, 2012, 07:00:20 pm
Quote
They also have a nice article on measuring speaker impedance with a multimeter.   
that one i have done before with some level of success 8) but not the antenna one.
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: sacherjj on March 15, 2012, 07:07:26 pm
Quote
They also have a nice article on measuring speaker impedance with a multimeter.   
that one i have done before with some level of success 8) but not the antenna one.

Audio frequency is at least a neighbor of DC frequency.  :)
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: wkb on March 15, 2012, 07:45:30 pm
Yep, a half-wave dipole should show exactly 75 Ohms. Anything else - it's broken, throw it away  :)

Oh... but you can bend it into shape to match Z0  :o
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: Ajahn Lambda on March 15, 2012, 08:28:58 pm
Most yagi style TV antennas are 300 ohm impedance and use a 4:1 balun to balance to 75 ohm coax.  I couldn't resist but to post something on there.

I saw that.  Nice.   ;D

Quote
They also have a nice article on measuring speaker impedance with a multimeter.  [/size] ::)

Ohferf*ckssake....!!!
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: G7PSK on March 15, 2012, 09:13:00 pm
I have a 70cm's aerial that is a square shape loop of 8mm dia aluminium tube resistance is near zero many tv aerials are of the same construction in the UK. I cannot imagine how they came up with their figures. But I had best throw out my long wire that appears to be open circuit (LOL). By the way have you read the related links like the one on measuring the output power on your TV aerial, they don't make it clear but I get the impression that they are talking about broadcast reception antennae, and are talking about up to 300 watts power.
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: AntiProtonBoy on March 15, 2012, 11:37:42 pm
Quote
5. Turn off the multimeter to preserve the batteries.
Great tip!
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: cybergibbons on March 15, 2012, 11:54:37 pm
Quote
6. Step away from the test equipment and never, ever write a guide again.

They missed this bit.
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: kj5cn on March 15, 2012, 11:57:21 pm
And, BTW, in that related link about measuring UHF antenna's output power (whatever that means) there's finally a proper definition of SWR - it's Signal Wave Ratio!  :-*
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: kd7eir on March 16, 2012, 12:04:21 am
All that time wasted reading and digesting the ARRL Antenna Book, all that wasted money on test equipment, and all I needed was a $5.00 DMM...

Now I have to spend my birthday weekend ripping out all my carefully buried cable, climbing towers, removing antennas, and starting over with the PROPER equipment...
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: kd7eir on March 16, 2012, 12:05:25 am
Quote
6. Step away from the test equipment and never, ever write a guide again.

They missed this bit.

Thanks for the laugh!
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: tekfan on March 16, 2012, 12:38:27 am
I bet you can measure the characteristic impedance of a coax exactly the same way!  ;D
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: vk6zgo on March 16, 2012, 12:54:49 am
It is sometimes a good idea to check the DC resistance of your feeder.----Easy if the antenna should look like a short,not so easy if you have to stand on the roof or up a stepladder or a tower, checking a dipole.
Of course,with the former,if there is a short at the other end ,you don't know if it's just the antenna,or a feeder short!

Strangely enough,there is something in the speaker impedance case--the resistance of a voice coil does give a fair idea of what the impedance may be,to the extent that you can tell the difference between different impedance speakers of similar kinds.Comparison between a 2" speaker & a 12" woofer may not work so well!  :D
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: sacherjj on March 16, 2012, 01:07:57 am
I'm trying to decide between:

(http://cdn.memegenerator.net/instances/400x/16373059.jpg)

or

(http://cdn.memegenerator.net/instances/400x/16373237.jpg)
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: janoc on March 16, 2012, 11:32:15 am
Guys, learn to read - the article title says:

"How to Measure UHF Antenna Resistance" (emphasis mine). And that is exactly what you would measure using that multimeter. The article never mentions impedance anywhere.

Is it useful to measure resistance on DC? Well, probably not so much, but you get what you asked for ...

Jan
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: T4P on March 16, 2012, 11:35:46 am
I'm trying to decide between:

(http://cdn.memegenerator.net/instances/400x/16373059.jpg)

or

(http://cdn.memegenerator.net/instances/400x/16373237.jpg)
Both's good  ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: sacherjj on March 16, 2012, 01:21:21 pm
Guys, learn to read - the article title says:

"How to Measure UHF Antenna Resistance" (emphasis mine). And that is exactly what you would measure using that multimeter. The article never mentions impedance anywhere.

Is it useful to measure resistance on DC? Well, probably not so much, but you get what you asked for ...

It is about as useful as writing an article about how to measure the resistance of a banana.  It tells me absolutely nothing useful about it.

If you are curious:

How to Measure Banana Resistance

Instructions
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: kj5cn on March 16, 2012, 02:48:42 pm
Quote
How to Measure Banana Resistance

Instructions

You should send all that to ehow.com - I'm sure they'd publish it right away!  :)

-Peter.
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: K9SPY on February 19, 2013, 11:16:28 pm
Ehow has emerged as the most worthless site on the internet. It is precisely because of how RELEVANT it shows up on sooo many many google searches that I consider google to be near useless anymore. Of course, not that any OTHER search engine IS...!
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: c4757p on February 19, 2013, 11:25:26 pm
  • Knock away and/or disconnect any mouth currently attached to the banana prior to testing.  If the banana peel is still attached, mock those who were trying to eat it in that form and remove.

Not sure why, but specifically that line cracked me up. Thank you  :)
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: ivan747 on February 20, 2013, 12:10:44 am

How to Measure Banana Resistance

Instructions
  • Knock away and/or disconnect any mouth currently attached to the banana prior to testing.  If the banana peel is still attached, mock those who were trying to eat it in that form and remove.
  • Determine what the preferred resistance is by referring to the grower specifications or the nutritional information for the banana. 
            Note: The label may only list Conductivity (Vitamin C)  This is the inverse of Resistivity.  You can then calculate the intended resistance by multiplying the Resistivity by banana length and divide by banana cross sectional area.
  • Turn on the digital multimeter. Set the multimeter selector dial to the Ohms readings and to the range which will include the desired resistance obtained from step two. The Ohms setting will look like a horseshoe or the Greek symbol for Omega.  This is because resistance is good luck.
            Example: If the desired resistance for the banana is 45 k Ohms, then set the selector dial to read a range that includes 45 k Ohms.  Or get an auto ranging DMM, you cheap ass.
  • Insert one test probe to the end of the banana and simultaneously insert the other test probe to the outer end of the banana.  Resist the urge to eat the banana like corn on the cob.  Read the display while holding the test probes in place to get the resistance reading.
            For example, if the desired resistance is 1 M Ohms, but the reading on the display is much less than 1 M Ohms, the banana will not function at its peak performance and should only be eaten cut up on cereal.
  • Turn off the multimeter to preserve the batteries.  Put the peel back on the banana to preserve the banana.
  • If the banana passed ohmic quality control (and you used ROHM certified, food safe probes) you may consume the banana.
            Note: Please follow all laws regarding proper peel disposal, as these are an environmental slip hazard.

Got my test setup ready  :)

(https://dl.dropbox.com/u/17276290/2013-02-19%2020.06.06.jpg)
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: jh15 on February 21, 2013, 01:41:20 pm
I usually blacklist it using my Chrome browser personal block list plugin. Quite an annoyance, these content farms.
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: grumpydoc on February 21, 2013, 04:26:05 pm
Surely you just plug the banana into the banana plug!
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: mamalala on February 21, 2013, 04:45:53 pm
Surely you just plug the banana into the banana plug!

Nah, you plug the banana plug into the banana, of course. What you propose is akin to plug a wall into a wall-socket. :D

Greetings,

Chris
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: zibadun on February 21, 2013, 08:39:47 pm
The author
"Laurie Brown has worked as a high school English teacher for the last several years and loves writing. She enjoys helping her students develop a love and appreciation for writing, reading, and literature. Laurie has a degree in education with a major in English. Currently she is a writer for eHow."

Yes, Laurie, the article is grammatically correct  |O

Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: c4757p on February 21, 2013, 11:15:02 pm
Nah, you plug the banana plug into the banana, of course. What you propose is akin to plug a wall into a wall-socket. :D

Like this (https://i.chzbgr.com/maxW500/4970710784/h607A8172/)? Seems somebody though it was a good idea...
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: ivan747 on February 22, 2013, 12:13:12 am
Surely you just plug the banana into the banana plug!

Is this compatible with plantains?
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: The Electrician on February 22, 2013, 05:31:02 am
And under the heading "What other people are reading", we find:

How to Measure an Antenna's Signal With a Multimeter

http://www.ehow.com/how_12130900_measure-antennas-signal-multimeter.html (http://www.ehow.com/how_12130900_measure-antennas-signal-multimeter.html)

where they use the ohms range on the Multimeter to "Measure an Antenna's Signal".!!!!
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: PA4TIM on February 22, 2013, 12:52:05 pm
The topic title is not so far off, it is indeed possible to measure the impedance of an antenna or other network with a multimeter.  ;)

But you need 3 of them and RF detector probes, google for impedance measurement 3 Voltmeter methode, this is from the time before impedance bridges and impedance ( RF-IV) meters, vector voltmeters  and VNAs. ( if I remember well it can also be done with 3 current meters)

I never tried it, but it is still on the to do list. But there are more ( forgotten) ways to to IV impedance measurements, like using a double ballanced mixer and voltmeters.
Title: Re: Who knew you could measure UHF antenna's impedance with a DMM?
Post by: mcufreak on February 23, 2013, 12:15:06 pm
Ha ha ha what a topic !!! :o

and that comment from 'sacherjj' about measuring banana resistance is one solid piece of entertainment.   :-DD

may be I should start learning about how to use a DMM again ..  :P