March 13, 2016

Testing Radios

Most of my communications equipment is procured inexpensively via used sources, especially common stuff like 27 MHz. CB transceivers. These acquisitions need to be tested to ensure they operate properly and to specifications. To accomplish this, some test equipment is needed.  Shown above is part of the initial test bed for checking radios. Its purpose is to test RF power output and receive sensitivity.  It's been my experience 90% of the time that as long as the radio can hear and transmit OK, the radio is either good or only needs a minor adjustment.

The test bed consists of the following items:
  • Signal Generator
  • RD Power Meter (Wattmeter)
  • Dummy Load
  • Power suppy
  • Connecting wires and coaxial cable jumpers.
  • RF adapters: N, BNC, PL-259/SO-239
  • Frequency Counter
Professional RF technicians use a multi-purpose piece of test equipment known as a service monitor that combines these functions.  Service monitors are expensive, even on the used equipment market.  For our purposes, separate pieces of test equipment cost less and accomplish the same functions. All the above equipment was purchased used over a period of a few years.  Some of it replaced previous purchases as the opportunity to upgrade presented itself.

Shown above are the signal generators.  The old Heathkit was the author's original piece of test equipment, and was later repaced by the HP8640B  (aka AN/USM-323).  The old Heathkit was good up to ~220 MHz. The HP goes up to 512 MHz. which makes it usable for testing up to the 70cm ham band.  90% of what I test operates at VHF or below so either of these are adequate.  Both get the job done, but the HP is a little more refined.

The SWR Meter is a hamfest find, probably 1970s vintage or so.  It's good for 100 watts up to 2 meters or so, which makes it fine for the CB I was testing.  Meters like this can be found new in the CB section of most truck stops.  It's no Bird, but it'll work for 90% of the ham gear you'll need it for.

Every broadcast transmit site I've ever visited had one of these stuck in a corner somewhere.  This is an original, oil-filled, Heathkit Cantenna dummy load.  Good up to 400 MHz. for 1 Kilowatt.  MFJ still makes a version of this, the MFJ-250.

 Finally we have the power supply.  This is a 30amp Astron RS-35A.  It'll provide enough current for testing a 100 watt 12V HF ham transceiver, which again is probably all you'll need initally.  If you were just testing stock CB radios, 2 meter mobile ham rigs, and QRP radios, you could probably get away with a lesser amperage rating.  Before finding this one, the bench had a 20 amp Astron supply on it.


A set of fused test leads ending on a pair of 30 amp rated clips are used for testing anything that doesn't have an Anderson Powerpole connector on it.  Note how there is a length of black electrical tape wrapped around one lead.  That's the negative connection.  With DC power, always mark your negative (- or black) and positive (+ or red) leads properly.  Actually, I should have used black primary wire for the negative lead, but didn't have any handy.  Thus the use of the black tape.

All of my equipment gets Anderson Powerpoles. This is the standard ARES/RACES power connector, and everyone in my group uses them for DC power connections.  This is the Powerpole test lead for the power supply.

Last but not least is the frequency counter.  It's just an old Radio Shack.  It's spec'd up to 1300 MHz. although it'll work higher than that.  There are now similar handheld models like the MFJ-886 that are spec'd out to 3 GHz.  I attach a rubber duck antenna to it, and place it on the bench somewhere near the dummy load.  There's enough RF signal for it to get a good frequency lock.

Not shown except in the periphery are assorted coax cable jumpers and adapters for hooking the radio up to the test equipment.

For testing purposes, this equipment set-up enables you to do the following:
  • Check receive sensitivity.
  • Check RF transmit power output.
  • Check RF transmit frequency.
  • Perform these tests on radio equipment operating from 160 to 2 meters.
Some of the equipment is actually spec'd out to go above the 2 meter ham band. The HP signal generator and frequency counter will do UHF.  By replacing the dummy load and wattmeter with something rated to =>500 MHz., you could extend the full capability up to UHF.  In a future blog post, I'll talk about upgrading this setup to UHF for testing 70cm ham band and GMRS radios.

4 comments:

  1. Excellent post, very helpful Thank you. Can you recommend a good old fashioned paper book to look for about repairing radios? Not much on amazon. Tnx

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  2. R R Looks perfect, TU

    ReplyDelete