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schematic for a field strength SWR METER @ 400-450 MHz ? — Parallax Forums

schematic for a field strength SWR METER @ 400-450 MHz ?

$WMc%$WMc% Posts: 1,884
edited 2009-08-24 06:14 in General Discussion
Hello All

I need a schematic for a field strength SWR METER @ 400-450 MHz. ?

My recent project involves the Parallax RX/TX modules part#s 27980 and 27981. I need to remote mount the ANT. to fit the module to the enclosure I have made for both units.· My concern is the ext. cable length and the affect it will have on the SWR of the module.

I have found some stuff on this topic on the net, But they want my E-Mail address to see their schematic...Then·SPAM me with junk Mail's.

·Thanks in advance

_____________________$WMc%________

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Comments

  • PhilldapillPhilldapill Posts: 1,283
    edited 2009-08-23 01:36
    Just a note, and an off topic one at that - Make a junk email address. I have one that I ONLY use for something like this where you don't care about what they send except maybe THIS ONE TIME. If they end up spamming you, oh well... It's not like you read it anyway.

    As for the SWR meter - Not a clue, but I hope that tip helps. [noparse]:D[/noparse]
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-08-23 01:40
    Here's a field strength meter schematic: www.qsl.net/n9zia/wireless/pics/fs_meter.png,
    and a project article for an SWR meter: ok1tic.nagano.cz/?download=uWave%20PSV%20metr_EN.pdf.

    -Phil
  • $WMc%$WMc% Posts: 1,884
    edited 2009-08-23 02:47
    PhiPi

    You are the Man..... Just what I needed!!!

    __Many_Thanks______$WMc%_________

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    The Truth is out there············································ BoogerWoods, FL. USA
  • HollyMinkowskiHollyMinkowski Posts: 1,398
    edited 2009-08-23 05:59
    How long is your feedline going to be?
    If it is very long you will get a lot of loss at UHF frequencies if you use the wrong sort.
    If just a few inches use the really tiny stuff as it's easy to work with...RG174 is what it's
    called if I remember correctly.

    The output of those rf modules is probably very weak, so it will take a really sensitive
    swr meter to read it or you may not get full scale...but even if the meter barely moves
    that is enough to trim an antenna for minimum swr with the meter set at maximum sensitivity
    and in reflected mode...just trim and look for the lowest meter reading.

    We use a meter called a Bird at work...it is good to about 3ghz and .1 to 10,000 watts.

    Probably just cutting the antenna to the correct length and making certain the feedline is
    properly connected will be good enough, the unit can probably take at least 3 to 1 swr
    continuously without harm...and the difference between a 1:1 match and 3:1 match will
    not be noticeable to you at all...especially with a very short feedline. If you are mounting
    the antenna atop a metal case and if it is large enough to provide a good ground plane
    then you can just use a 1/4 wave length of stiff wire for your antenna...if it is cut exactly
    right and is on a suitably sized metal base you can get as low as about 1 to 1.3 swr.

    It seems like a lot of work to make an swr meter for taking a single reading. Do a google
    search on the name of the town where you live and 'ham' or 'amateur radio' and you can
    find some local hams. One of them will have a meter and will be glad to have you over to
    take your readings. He will also probably have some short lengths of RG174 in the junkbox
    he will give you smile.gif If you have a ham's callsign you can look up where they live and do a
    phone number search on one of the phone book search engines to get the phone #.

    eham.net has a ham call search engine you can use.
    Look here for info about making simple ground plane antennas.
    www.google.com/#hl=en&source=hp&q=1%2F4+wave+antenna+plans&aq=f&aqi=g10&fp=c9fe100d9e542c1e

    We use a lot of antennas where I work. Mostly 2 types.. discones and microwave dish antennas.
    The discone is a great vertically polarized antenna that has a low swr across a very wide spectrum
    and they are easy to make yourself for frequencies 400mhz and up... there is no need
    to even test the swr with these. The dish antennas we use are for 2.5 and 5ghz and cost just 50.00
    They have 24db gain (that's a LOT)

    discone_12-lrg.jpg
    andrew_2400_grid_antenna.jpg

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  • Tracy AllenTracy Allen Posts: 6,666
    edited 2009-08-23 06:35
    Here is another SWR circuit that I found intriguing due to its construction as a resistive bridge. The text has good insight into the nature of SWR.

    ludens.cl/Electron/swr/swr.html

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    Tracy Allen
    www.emesystems.com
  • HollyMinkowskiHollyMinkowski Posts: 1,398
    edited 2009-08-23 06:59
    @Tracey Allen

    LoL, I saw the emesystems url and since EME is an abbreviation for using the moon
    for communications by bouncing rf signals off of it I surfed over to see all the cool
    antennas and stuff smile.gif

    images.google.com/images?hl=en&source=hp&q=eme+moon&btnG=Search+Images&gbv=2&aq=f&oq=&aqi=

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    - Some mornings I wake up cranky.....but usually I just let him sleep in -
  • LeonLeon Posts: 7,620
    edited 2009-08-23 15:15
    Field strength and VSWR are completely different, and two instruments are required.

    Leon

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    Amateur radio callsign: G1HSM
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  • Tracy AllenTracy Allen Posts: 6,666
    edited 2009-08-24 04:56
    I've gotten emails from people asking about Earth-Moon-Earth (and about philately, too). I have some of that rigid coax, 50 ohms and small diameter, that I've used to make small DF (direction finder) loops down to 1 cm diameter. They make great probe for mapping fields close up, or EMI probes. That is about the extent of the weird or cool antennas that I use!

    Most SWR meters attempt to be loss-less over the band of interest by use of a directional coupler or some such magic. So they can stay installed in the feed line even at high power levels. That link I pointed to inserts the Antenna feed as one arm of an otherwise resistive 50 Ohm Wheatstone bridge circuit, and measures the voltage and phase difference across the arms of the bridge. Terribly lossy, so one would not usually want to leave it in the line, only for setup of an antenna at low power. It should also be useful for some purpose where VSWR reflects some physical quantity to be measured.

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    Tracy Allen
    www.emesystems.com
  • $WMc%$WMc% Posts: 1,884
    edited 2009-08-24 06:14
    I Thank You all

    This is great info. I never expected to get this much input on a " RF related topic " in a micro controller forum. This is great!!!
    I have downloaded a ton of info from the sites "You'll" have provided.

    The ext. cable is about 3/4"·long. Just enough to pass through the enclosure and keep a water tight seal. (a bulk head connector)


    __Thanks Again_________$WMc%_________

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    The Truth is out there············································ BoogerWoods, FL. USA

    Post Edited ($WMc%) : 8/24/2009 6:41:10 AM GMT
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