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Ham Radio related projects, ideas,
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Monday, November 24, 2003
Found an Omega-T Antenna Noise Bridge in my junkbox. Cleaned it up, put in a fresh battery and set about to test a new 160M dipole. Can't find a null at the cut frequency or anywhere nearby. Anyone have a schematic of this old bridge? Tuesday, August 12, 2003
K1KP's Procedure to measure IF filters Equipment needed: RF signal generator which covers the filter’s range of frequencies. 50 ohm output impedance. Step attenuator. This will be your primary signal level standard. Should have at least 70 (preferably 90+) db range and be 50 ohm impedance. HF general coverage receiver. This must cover the filter’s range of frequencies. It should have a means of turning off the AGC and using manual RF gain control. It will be your primary frequency standard, so a digital display is recommended. It should have an analog S-meter. (Optional) Audio AC voltmeter (or Scope) This can be used in place of the receiver’s S-meter for better accuracy. Equipment setup: The basic setup is to feed the output of the RF generator thru the Step attenuator into the filter. The filter output is fed into the HF receiver. If you are using the Audio voltmeter, connect it to the receiver’s speaker output. Filter Termination: For best results, the input and output impedance of the filter should be matched according to the filter specs. This may be difficult as the filter impedance is not always known. In some cases it may be printed right on the filter. Also, some filters are designed to have their inputs and outputs tuned with a small value capacitor. The filter impedances can be matched to 50 ohms by using a series resistance. Example: Heathkit filters have Zin = Zout = 2000 ohms. The RF generator has a 50 ohm output impedance. Use 1950 ohms in series with input and output so the filter ‘sees’ 2K ohms on both ports. Also, a good ground into and out of the filter is required to get accurate results. A small impedance in the ground can lead to erroneously high stopband ‘blowby’. In Situ Measurements: The best results can be had by using the filter in the rig where it will be used (called Rig Under Test or RUT in the following). This is because the termination and grounding issues are taken care of for you. In this case, you can connect the RF generator to the input of the RUT and set it to any frequency the RUT can receive. The RUT will convert the RF generator frequency to the passband range of the filter. Connect the HF receiver used for measurement to a downstream stage to pick off a signal after the filter. Be sure to disable the AGC in the RUT, and set the RF generator output level and RUT RF gain control so the RUT is not overloaded. Measurement technique: The HF receiver should be in CW or SSB mode, using as wide a bandwidth as possible. Tune the generator and HF receiver to peak up a signal and find the peak of the filter’s passband. Set the step attenuator to around 70 or 80 dB of attenuation. Adjust the generator output so you have a reading of S9 or so on the receiver. The procedure for plotting the filter response is to make a series of signal level measurements at the frequencies of interest. These measurements will be relative to a single reference, so you will be able to see the filter’s passband characteristic but not it’s absolute insertion loss. There are two basic ways to make these relative measurements. The first is using the S-meter of the HF receiver to indicate a fixed signal level. For this technique, the AGC in the HF receiver is on. At the filter peak, record the S-meter reading. This is your ‘reference level’. Next set the RF generator to the frequency you wish to measure. You can determine exact frequency by zero beating and reading the digital display from the HF receiver. Each time you change the RF generator, retune the HF rig to give the same beatnote (i.e. the HF receiver must track the generator frequency). This also prevents the filter characteristics of the HF receiver from affecting the results – all the measurements are at the same point in the HF receiver’s IF. As you move the RF generator signal away from the peak, the S-meter reading will decrease. Keeping the RF generator fixed at the frequency where you want to make a measurement, decrease the attenuation on the step attenuator until the S-meter comes back up to the recorded reading ‘reference level’. Now you can read the change in filter attenuation by noting the change you made in the step attenuator setting. The second method is to use an audio AC voltmeter instead of the S-meter to measure the reference level. In this case, turn off the AGC for the HF receiver and use manual RF gain control to make sure the HF receiver is not overloaded. Measure the audio output from the receiver with the AC voltmeter. This is your ‘reference level’. Change the RF generator and HF receiver frequency together. Next adjust the step attenuator to bring the level back up to the reference level. Now read the filter attenuation on the step attenuator as above. Caveats: At the extremes of measurement, you need to be sure you are not overloading the HF receiver (or the RUT). Do this by making a 3-6 db decrease in the step attenuator, and checking that signal levels at the measurement point increase appropriately. If they don’t then you may need to readjust the RF generator and start the measurements over with a new ‘reference level’. Another Way A very convenient way to measure filters is using a DSP receiver such as the 756 Pro series with a PC equipped with a sound card and audio spectral anaylsis software (shareware). A good program to use is Spectrogram from Visualization Software. The sound card software is used to measure and plot the audio output from the HF receiver. As long as the bandwidth of the HF receiver is wider than the filter you want to test, you can easily sweep the input to the filter while observing the output on the computer screen. In this technique, you need to turn off the AGC in the HF receiver so it doesn’t affect the signal level measured by the sound card. This technique could also be used to measure the passband of a RUT directly, as long as the AGC can be turned off. Tony K1KP Saturday, June 14, 2003
Recently obtained a Collins military transmitter, with Navy nomenclature of TDO. Not much reference to this rig on the web. Ft. Gordon has this: BC-459* Transmitter, 2-18 MHz, 250/200 W, U/W RM-20; TM 11-812. Later models same as Navy TDO. There was a reference to this rig on the usenet boatanchors list: Subject: Re: Collins BC-460 View: Complete Thread (2 articles) Original Format Newsgroups: rec.radio.amateur.boatanchors Date: 1999/11/01 W8ak wrote: > I found an old Collins BC460 transmitter today... > Is it worth bringing home? I sure wish *I* had found it ;-). It's a WW-II Army Airways transmitter, used in HF air traffic control. It is patterned after the Navy TDO. It covers 2-18 MC, 250 watts AM/CW/RTTY, VFO or crystal control in early models, VFO only in late models. Runs on 220VAC single phase. Better then a BC-610 in my book! 73 OM DE Dave Stinson --- It is a Collins 18F Model 6, looks like a big brother to the ART-13. Has a pair of 813's in the final, 10 channel autotune, 7foot cabinet. Anyone know for sure what the Army designation for this rig really is? Image here. Friday, June 13, 2003
Who Provides your long distance, regional & local service? To check to see who your Long Distance telephone service provider is, is very simple. All you have to do is call the following number from your home telephone. This is an automated system run by the FCC which is kept current by the local telephone company (this is why they are allowed to charge for the service of switching Long distance providers because they are required to do the database updates in a timely manner). The number to call is (700)-555-4141 To check to see who your Regional (in state or intraLATA) telephone service provider is, is very simple also. All you have to do is call the following number from your home telephone. This is an automated system similar to the system for the long distance except it is specific for each area code. This system is also updated regularly but if the change is made concurrently with a change in Long Distance telephone service provider then there is no charge. The number to call is your area code-700-4141 Friday, April 04, 2003
Found an AS-3345/G antenna. This is a stowable VHF dipole type assembly, BNC antenna connector. What equipment did this antenna go with? Thursday, April 03, 2003
Radar Systems - C Band Radar type: AN/APM-132. Freq. Bond: Recv 8.7-9.6, trans. 9325± 15MHz. Pwr output: 100 watts. Magnetron type: L3756. Pulse width: .5µs D.C., .005. Antenna: Dipole Pwr input: 120V, 3 ph, 400 Hz. Mfr. Motorola I have an Inclinometer set part of the APM-132 unit. X-band, presumably. Is this useful to a radar equipment collector/historian? Friday, March 21, 2003
Dick Smith ESR Meter and LCR-40 arrived. A quick check of the LCR shows promise that it will be very useful. It looks like a small mpeg player. The only surprise to me so far is the very short test cables. Is that to maintain easier calibration and compensation for stray capacitance of the leads? The ESR Meter is a KIT that has to be assembled. Perhaps this weekend I'll get to it. Friday, March 14, 2003
A common problem with restoring vintage equipment, both solid state and tube type, is determining which capacitors need replacement in the subject equipment. Some propose that all caps be replaced in a piece of gear that is 40 or 50 years old because the construction methods of the capacitors at the time guaranteed that the components would fail with age and environment. Paper caps for example were known to absorb moisture which eventually led to 'leaky' conditions. The cap acts more and more like a resistor. In a tube circuit with high voltage that could spell big trouble for related components like dropping resistors, output transformers, power sources, etc. Others maintain that wholesale replacement of all caps is just asking for unnecessary work and trouble debugging the entire system, especially where circuits have low voltage and the caps are overrated. I've ordered a Dick Smith ESR meter to do some sample testing with selected equipment that is waiting on workbench space. I have an early Gertsch WWV receiver (solid state) with easy access to all the caps, a junker SX-28 receiver carcass, a Drake 2B, a Viking I transmitter, some R-389 IF modules. Also ordered an Atlas LCR meter to see if it will help me measure and sort thousands of capacitors that I've accumulated for spares. A touch of color blindness makes it difficult for me to read older cap values. Hope this little marvel helps. Both of these meters are advertised in Nuts and Volts magazine, which I have recently rediscovered at my local Barnes and Noble. Friday, March 07, 2003
CQ CQ CQ de W5OR BT Starting Blog K By way of introduction, here are a few radio related sites I fool with: www.militaryradio.com www.radiohome.com www.r-389.com |
