rfradioconsult
Member
Those of us of old school, have fond memories of a Grid Dip Oscillator. I still have my Millen with the coil set and stand.
No SWR meter/Antenna Analyzer/..., now what?
- Thread starter needairtime
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- Joined
- Jun 13, 2018
- Messages
- 869
I don't hear much about people today using Grid Dip Meters-- but in their day, those days of home built tube equipment especially, they were as as common on a 'homebrewer's" bench as his/her soldering iron. I grew up with them about me from my first breath-- my father and his father were great builders of anything radio.
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Years later I taught a under graduate course in antennas- their theory and designs. I did not start out with exotic measuring protocols like time domain reflectology....but with the historical stuff like Lecher wires, V/SWR Bridges, Noise Bridges.... and, of course, the use of Grid Dip Meters.
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Of all the ways to measure antenna parameters, I confess my admiration for those that could get meaningful data out of their use. I had to admit to my students that there were artists out there that could - but it wasn't going to come from me- irregardless what their course syllabus said.
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I add'd that the coupling of these meters was critical; useful only for resonant circuits that absorb energy sufficient at the desired frequencies, and that the Q had to be fairly high.
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Since the combination of both antenna and feedline often presents a series of harmonic results, the often-many readings could lead to erroneous conclusions about the true resonance of an antenna.
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I concluded my failed antenna labs demonstrations with the admonition that Grid Dip Meters, in the hands of those experienced with them was fine,-- but not me. They (the students) should regard them as a basic frequency meters- and good luck with their antenna projects- we'll have a quiz Friday...
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Lauri
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Years later I taught a under graduate course in antennas- their theory and designs. I did not start out with exotic measuring protocols like time domain reflectology....but with the historical stuff like Lecher wires, V/SWR Bridges, Noise Bridges.... and, of course, the use of Grid Dip Meters.
.
Of all the ways to measure antenna parameters, I confess my admiration for those that could get meaningful data out of their use. I had to admit to my students that there were artists out there that could - but it wasn't going to come from me- irregardless what their course syllabus said.
.
I add'd that the coupling of these meters was critical; useful only for resonant circuits that absorb energy sufficient at the desired frequencies, and that the Q had to be fairly high.
.
Since the combination of both antenna and feedline often presents a series of harmonic results, the often-many readings could lead to erroneous conclusions about the true resonance of an antenna.
.
I concluded my failed antenna labs demonstrations with the admonition that Grid Dip Meters, in the hands of those experienced with them was fine,-- but not me. They (the students) should regard them as a basic frequency meters- and good luck with their antenna projects- we'll have a quiz Friday...
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Lauri
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needairtime
Member
I now own an unknown condition CB SWR meter, courtesy of a thrift shop.
Now to experiment with it... I do have CB equipment so I guess that gets experimented with first, at least until I can get 2m going to some worthwhile frequency (grr... still trying to get my colpitts oscillator working. I've had to nerf down my grandeur goals quite a bit due to feasibility issues with what I have to work with.)
Might go ahead and see what it does with UHF too, hopefully won't blow up the radio...
Now to experiment with it... I do have CB equipment so I guess that gets experimented with first, at least until I can get 2m going to some worthwhile frequency (grr... still trying to get my colpitts oscillator working. I've had to nerf down my grandeur goals quite a bit due to feasibility issues with what I have to work with.)
Might go ahead and see what it does with UHF too, hopefully won't blow up the radio...
Needairtime-
If you can make a dummy load out of resistors, even if it is not exactly 50 ohms, you can try the meter at low power on almost any frequency by calculating the 'expected' SWR. Adding coax and playing with a smith chart (if you know the velocity factor) can also let you see if the meter is giving 'reasonable' readings.
Have fun
If you can make a dummy load out of resistors, even if it is not exactly 50 ohms, you can try the meter at low power on almost any frequency by calculating the 'expected' SWR. Adding coax and playing with a smith chart (if you know the velocity factor) can also let you see if the meter is giving 'reasonable' readings.
Have fun
- Joined
- Jun 13, 2018
- Messages
- 869
" ...... Might go ahead and see what it does with UHF too, hopefully won't blow up the radio. "
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I don't think that would damage anything- but using a CB SWR meter on UHF is not going to give you any meaningful measurements. Their physical design, not to mention the physics involved --is too sloppy to work on anything much above 150Mhz.... and I'd be careful trusting anything up that high.
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However, if you have a CB meter that you'd care to sacrifice, tear the guts out of it and construct your own thru-line circuit for UHF. There are plenty of designs in radio handbooks; and you can use the case, the meter movements, pots, switch's etc. in your new instrument. Plus you'll see first hand what it takes to get meaningful readings at these higher frequencies--- comparing them before and after..........
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Lauri
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I don't think that would damage anything- but using a CB SWR meter on UHF is not going to give you any meaningful measurements. Their physical design, not to mention the physics involved --is too sloppy to work on anything much above 150Mhz.... and I'd be careful trusting anything up that high.
.
However, if you have a CB meter that you'd care to sacrifice, tear the guts out of it and construct your own thru-line circuit for UHF. There are plenty of designs in radio handbooks; and you can use the case, the meter movements, pots, switch's etc. in your new instrument. Plus you'll see first hand what it takes to get meaningful readings at these higher frequencies--- comparing them before and after..........
.
.
.
Lauri
needairtime
Member
I finally got this #*(@&ing thing open, the manufacturer used loctite or rather probably some other CA glue on the threads. The main reason for opening was due to the adjustment knob being really scratchy, I sprayed some contact cleaner and it works better now.
However yes I note that there is the through line circuit, and a diode tapping off of opposite ends of each of the side lines. The lengths of the wires don't match at all, which probably would give me misleading results at anything shorter than probably 10m.
What was interesting is that the through line circuit is huge. Comparing it to the built-in through line SWR detector on the radios, this thing is ridiculously large. Is it needed to be this large, or is the size solely needed to drive that analog meter with enough current? (I should measure how many µA is needed to get a FS reading on that meter just for curiosity sake...)
I was thinking about evening up the two sides with identical diodes or at least ones that I know the characteristics of, and adding op amps to a microcontroller...and have the microcontroller calculate SWR without the need for that knob or the FWD/REF switch...
However yes I note that there is the through line circuit, and a diode tapping off of opposite ends of each of the side lines. The lengths of the wires don't match at all, which probably would give me misleading results at anything shorter than probably 10m.
What was interesting is that the through line circuit is huge. Comparing it to the built-in through line SWR detector on the radios, this thing is ridiculously large. Is it needed to be this large, or is the size solely needed to drive that analog meter with enough current? (I should measure how many µA is needed to get a FS reading on that meter just for curiosity sake...)
I was thinking about evening up the two sides with identical diodes or at least ones that I know the characteristics of, and adding op amps to a microcontroller...and have the microcontroller calculate SWR without the need for that knob or the FWD/REF switch...
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