[hfsig] Antenna analyzer kit

Bruce Raymond bruce at raymondtech.net
Mon Mar 25 17:20:11 EDT 2019

Hi Guys,

I ran into trouble with email size on the last blast-o-gram. I don't 
know if the picture came through, so I'll send the email again with a 
much lower resolution picture. Please ignore this email if the previous 
email made it through.

Again, any and all feedback is welcome.

73 Bruce/ND8I

Bruce Raymond wrote on 3/24/2019 9:52 PM:
> Hi Ed and Ricardo,
> Thanks for the feedback. Much appreciated.
> My concept for the antenna analyzer is to combine an Arduino 
> (computer), Si5351a board (oscillator, frequency set by the Arduino) 
> and an LCD diisplay. The oscillator drives a simple resistor bridge 
> with the antenna as one of the legs of the bridge. I'm using an AD8307 
> log amp to measure the voltage across the bridge. I've attached a 
> picture of the prototype analyzer.
> The idea is to sweep a range of frequencies to find the resonant point 
> and calculate and display the VSWR at that frequency.
> Initially I thought that I could also measure the bandwidth where the 
> VSWR of the antenna is 2:1 or less. I wasn't able to get that to work. 
> After some reflection I realized that there are reactive components 
> once you get away from resonance that make measuring VSWR impossible 
> with a resistive bridge.
> I've tried to keep the operation simple. After turning the power on, 
> you turn the knob (rotary encoder) to set the center frequency of the 
> sweep; that's shown as "IN:" in the photo. Each click of the knob 
> changes the frequency by 1 MHz. Then analyzer sweeps from 0.5 - 1.5 
> times the center frequency. In the case shown (center frequency = 12 
> MHz), the analyzer sweeps from 6 MHz - 18 MHz. After you set the 
> center frequency you push the knob to start the sweep. It's fairly 
> quick - takes about 2 sec to do a sweep. The analyzer should work over 
> a frequency range of 1-30 MHz. It's possible that the range can be 
> expanded.
> My objective was to make something that's portable and relatively 
> cheap, sorta trading accuracy for money. I think it will be accurate 
> within 5%-10%. Actually, the frequency measurement seems to be 
> accurate within 10 KHz and the SWR within ~0.1 at resonance.
> This is an open source project, so I'm planning on releasing the 
> source code into the wild. I'm planning on shipping an preprogrammed 
> ProMini with the kit so that you don't have to do any programming or 
> loading of software.
> I have two AD8307 chips in the design. These are smd chips that I'm 
> going to have presoldered to the pcb so the builder doesn't have to do 
> that. Everything else is through hole/parts with leads. There are a 
> couple of other smd chips shown on the prototype. These were part of 
> another project (mag loop autotuner) and not part of the antenna analyzer.
> I'm planning on having the analyzer available to show at 
> Hamvention/Dayton in May and have kits available through TAPR 
> hopefully in July - if I can get the documentation ready  ;-)
> I'm interested in everybody's thoughts and comments. Any and all 
> feedback is welcome.
> 73 Bruce/ND8I

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