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.

Minivna