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Author Topic: Aligning QRP Kit with only an Oscilloscope and VOM? No other Tranceiver.  (Read 4005 times)
KB3VIS
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Posts: 15




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« on: July 24, 2012, 08:23:37 PM »

Hey guys,

I am about to finish building a Ten-Tec 1320 QRP Kit and I'll need to align it.  Unfortunately I don't have access to another transceiver, SWR meter, or RF generator.  I have a digital oscilloscope and VOM and not much else.  I figured I would attempt to make a quarter wave monopole for the 20m band and hopefully find something on the air waves to use.  How likely do you think I'll have success with this?  Does anyone have any suggestions for me?  Thanks
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K7KBN
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« Reply #1 on: July 24, 2012, 09:49:03 PM »

I'm not at all familiar with that kit, but I would certainly recommend a known-good dummy load rather than building a dipole the characteristics of which you won't know until you check it with your SWR meter...oops!  You don't have one.

If the 'scope is calibrated properly, you can use it as a pretty close frequency indicator, and you mentioned a "VOM".  Is that a digital or analog meter?  If you're going to be peaking and nulling circuitry, an analog meter will be your best friend.
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73
Pat K7KBN
CWO4 USNR Ret.
KB3VIS
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Posts: 15




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« Reply #2 on: July 24, 2012, 10:09:58 PM »

The VOM is digital.  The oscilloscope also has a frequency counter built in, so I can get good values into the dummy load I guess.
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KB3VIS
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Posts: 15




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« Reply #3 on: July 24, 2012, 10:27:23 PM »

On that note, if I do end up buying an SWR meter, how cheap should I go?  I'm not looking for an analyzer, just a simple analog needle meter.  Any recommendations?
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K8AXW
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« Reply #4 on: July 25, 2012, 08:42:04 AM »

You should be able to get an HFSWR meter for $20.00.  Radio Shack sells them.  Just make sure it is HF and not VHF.... and makes sure the meter will handle the power ....especially low power.  Getting one to give a full scale reading at low power is a problem.  Consider a CB SWR meter because they are designed to operate at 5w or less.

No need to spend big bucks on this meter.
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PA0BLAH
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« Reply #5 on: July 25, 2012, 09:04:54 AM »

What the heck is the sense of an SWR meter?
You measure the SWR on an antenna load.

What he needs, I guess because he doesn't provide a link to a schematic diagram and a manual, is a dummy for a few watt.

Take a pair of carbon 1W or 2W  resistors 100 ohm en put them in parallel, that is your dummyload of 50 ohm.

Take a diode and a capacitor 1000 pF not critical (10000 and 100 will do also) in series,put them over the dummyload. Capacitor at mass side (case cabinet) put a 100 K resistor parallel to capacitor.  and measure with your DC VOM the voltage over the capacitor. Call that value U.
That is the peak voltage, effective is factor sqrt(2) smaller . Squared U squared/2. Hence dissipated power U squared/100

Example : You measure U= 10 V DC--> power is 1 watt. When you measure 20V DC the RF power is 20*20/100=4 watt

Never spent a dollar on some cheap SWR meter. That is one of the devices you can easily build yourself. Every ARRL and Antennahandbook  of ARRL or RSGB shows you how to do that. And for alignment you certainly don't need it. It will show that the SWR is 1 in your dummy  and when not throw it away, or sell it to some sucker on e bay.


Here you have a link:
https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnxndzZpdGp8Z3g6OGM4NDg5ZTk3MWZlYWZk

Make the dummy of 2 pc 100 ohm 2W resistors, and the powermeter from your VOM as said. A signal source is easily hooked up, with a transistor and a aircoil of a few turns, your scope-counter works as "HF receiver with digital frequency read out"

In short, you get a lot of dogshit as advice here from others, and you better listen to me, all will be without extra cost  done with the excellent equipment you have available.

Any questions left?

Bob
« Last Edit: July 25, 2012, 09:45:30 AM by PA0BLAH » Logged
PA0BLAH
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Posts: 0




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« Reply #6 on: July 25, 2012, 11:30:36 PM »

BTW Uou don't even need to chase and download that software program for audioanalysing mentioned in that link, because that is for guys without access to a frequency counter that you have available in your scope.
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K8AG
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Posts: 352




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« Reply #7 on: July 26, 2012, 09:18:38 AM »

You should be able to get an HFSWR meter for $20.00.  Radio Shack sells them.  Just make sure it is HF and not VHF.... and makes sure the meter will handle the power ....especially low power.  Getting one to give a full scale reading at low power is a problem.  Consider a CB SWR meter because they are designed to operate at 5w or less.

No need to spend big bucks on this meter.
The problem with the CB SWR meters is that they generally become significantly less responsive below the CB band.  Down at 20M they would still work but not as well.

I would recommend looking at the Hendircks QRP kits (Tayloe SWR Indicator/Bridge Kit).  This type of indicator provides lower tuneup radiation and some protection from high SWR antennas.  They are pretty much standard issue for QRP ops.  You can even find the circuit diagram and build one.  Very handy.

Good luck.

73, JP, K8AG
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K8AXW
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« Reply #8 on: July 27, 2012, 09:10:45 AM »

AG:  You're absolutely correct on all points!  My suggestion(s) were made simply to get this guy going.  There's nothing more frustrating that to complete a project only to find that you have to scrounge up something to align it!

I just went through this situation in which I needed a frequency counter to set 3 circuits.  It took me a year to get the counter.  That sucks! 

While a CB SWR meter in fact does lose sensitivity below 27MHz, it still can be used as an indicator.  However, he still should have a dummy antenna instead of an antenna to initial tuneups.  There again Radio Shack sells CB dummy antennas cheap that will work.

I built my QRP dummy load (as PA0BLAH suggested) because I happened to have a drawer full of 2w resistors.  In case VIS doesn't have said drawer full of resistors, the name of the game is speed!   
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K8AG
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Posts: 352




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« Reply #9 on: July 27, 2012, 09:12:52 AM »

I'm still building up and tearing down test equipment of all sorts.  But then I have only been at this for 50 years or so.  Someday I'll get it done.  Smiley
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PA0BLAH
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« Reply #10 on: July 27, 2012, 09:39:52 AM »


I just went through this situation in which I needed a frequency counter to set 3 circuits.  It took me a year to get the counter.  That sucks!  

Make one with a microcontroller, and when you are living in rural nowhere buy a kit "numerical dial" as suggested in a recent posting, which is a counter with IF offset. Can be used to extend your QRP set. Hendricks QRP sets, something like that.

You have a black Japanese cabinet with a 100 small buttons as rig, I suppose, and you are proud owner I suppose, that emits frequencies related to the dial, you can use these as know frequencies, or a griddipper, checked with your Japanese appliance dial reading.
Quote

I built my QRP dummy load (as PA0BLAH suggested) because I happened to have a drawer full of 2w resistors.
[/quote]
Mil.Spec. I suppose.
Dutch translated proverb: Who transport apples by ship eats apples.

Put a pair in an envelope en sent that guy a few. That is ham spirit. Not hamspirit is telling that PA0BLAH produces only dogshit and warn other writers they can better not react.
Quote
In case VIS doesn't have said drawer full of resistors, the name of the game is speed

Nothing about that, he didn't even react till right now, has a link to read, and can easily wait, only children have the strong desire of instantaneous desire fulfillment.
Four 1 W resistors of 220 Ohm will do equally well, or 16 resistors 820 ohm 0.25 W
« Last Edit: July 27, 2012, 09:49:08 AM by PA0BLAH » Logged
K8AXW
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Posts: 3958




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« Reply #11 on: July 28, 2012, 08:50:24 AM »

 Roll Eyes
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PA0WV
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Posts: 137




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« Reply #12 on: July 28, 2012, 09:28:22 AM »


Four 1 W resistors of 220 Ohm will do equally well

Two of 180 ohm and two of 220 ohm 1W all in parallel will even do better.

For alignment of the kit of KB3VIS is no SWR meter  required, when you need an SWR meter build one yourself.

It is also possible to build one with output independent of frequency for the HF bands with a tandem directional coupler.  

I designed and build a meter indicating radiated power,
SWR and PEP all on linear scales, with the Phoenix3
link  17 from top on my website http://pa0wv.home.xs4all.nl/zelfbouw.html the tandem directional coupler is part of it.
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Using an appliance without CW is just CB
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