I can't work out how the bridge circuit works - I think it is R28, R29, R30 and U15 but it doesn't seem to be connected in the way I would expect. Pin 1 & 2 should be ground and pin 3 is the unbalanced signal. The unbalanced side of several of the GH marked Minicircuits baluns seem to be connected inconsistently compared to the datasheet. Maybe this is all wrong, but here are some things I noticed: RFFC5071 mixer 1 output is 30MHz minimum but goes directly into a 4.8kHz max ADC. I just spent some time figuring out how I think it works. One other thing I can add is if you have a sweep generator (any cheap DDS off eBay will work) a directional coupler or resistive bridge, and an RTLSDR dongle, you can get pretty close results to many of the low cost VNA units out there. A good indicator of a problem is if they offer a broadcast band filter as an option. The bottom line is I would check any of the VNA units you are considering for problems with broadcast stations.
In a perfect world, this setup would work fine but the 8307 is reading not only the reflected power from the signal generator, but all the harmonics as well as any broadcast stations, amateurs, etc. Many of the low cost VNA units use a signal generator with a directional coupler and an AD8307 for the returned power measurement. So no mater what you buy, I suggest you find something that does precise power measurements with some type of filtering. Trying to plot my off center fed dipole and I had peaks and dips and all kinds of problems.
The problem I had with simple VNA circuits as I tried to build my own before settling on the N2PK VNA, was that they were very broad-band and if you had any local broadcast stations (and I do) then it was worthless. The directivity generally isn't as good as the Wiltron bridges/autotesters, but they can be found for $50 or so, with patience. If you've got an RF detector for the range, another option would be a directional coupler.
Sweep isn't really necessary as you can step through the frequencies of interest. You'll also need a signal generator that covers the frequencies of interest, if you don't have one already. An "SWR Autotester" includes a detector, while an "SWR Bridge" does not. The higher directivity ones (40 dB) covering up to a couple GHz can be had for $200 with patience. If you've got 40 dB of directivity you can probably measure return loss down to about 20-30 dB See, for example, Wiltron (now Anritsu) makes several models. For tuning antennas, you often just need the magnitude. One option to consider is a VSWR bridge which will give you magnitude, but not phase. I would too.except that the asking price is generally out of my budget.
0 kommentar(er)
