Visually Tune Your HF Antenna Using An Oscilloscope And Signal Generator

Lots of readers are into toying around with RF and ham radios. One thing that is always of concern is tuning the antenna. New equipment is never cheap, so whenever another option comes along that uses existing test gear it gets our attention. [Alan Wolke] aka [w2aew] covers a process he uses to tune his HF antenna using a signal generator and oscilloscope.

The process is more of a teaching aid than a practical replacement for commercial equipment mostly because proper signal generators and oscilloscopes are large items and sometimes not available or affordable. That said, if you do have such test gear you only need build a simple breakout board containing a form of wheatstone bridge where the unknown Rx is the antenna. Two oscilloscope probes are connected across the bridge balance nodes. Some special care needs to be taken matching probe cable length and 50 ohm input impedance to the oscilloscope. A couple of 1K probe coupling resistors are also needed to prevent affecting the impendence at the hookup points. Once the selected signal is injected you can adjust an antenna tuner until the two voltage waveforms match on the oscilloscope indicating your antenna network is tuned to 50 ohm impedance with no reactance.

Being able to tune your antenna visually can really help you understand what is going on in the turning process; matching not only input impedance but also phase shift indicating inductive or capacitive reactance. Join us after the break to see the video and for information on what’s presented in the second part of [Alan’s] presentation.

The lesson ends at 8:50 but continues ten seconds later with a part 2 presentation “Estimating the complex impedance of the antenna”.

15 thoughts on “Visually Tune Your HF Antenna Using An Oscilloscope And Signal Generator

  1. This is very cool. I love it when general purpose lab equipment can be substituted for special purpose gear. Would a pair of BNC T-adapters connected to the incoming cables, the scope inputs, and a pair of 50Ω terminators work in place of the 50Ω thru-terminators?

    1. Oh man, that’s an idea. My hacked together coat hanger antenna is in the attic. I get great reception except for sometimes on PBS. I hesitate to try moving it because it’s such a mess and a pain to climb up there. If a buddy could watch the meter while I move it that would be ideal.

      +1 on somebody explaining how to do this.

    2. You’d need a specrtum analyzer to really fine-tune your OTA TV antenna positioning (spectrum analyzers have been featured on HAD before, though I’m at a loss for links at the moment). Heck, you could almost use an SDR dongle as a sort of spectrum analyzer on a channel-by-channel basis… (you’d just have to look up the real channels for the stations you wish to receive in order to determine their frequencies)… not ideal, but certainly functional with patience.

      But, this *would* be very useful in matching your OTA TV antenna to the TV tuner input… if you had stations you want to receive and they’re iffy, matching the antenna might just make enough of a difference to bring them in. In short, it wouldn’t help much, but it might just help *enough*.

      Good luck!

  2. Hacks like this are why I come to HAD, Could be very useful and one of those “Gee wiz, that makes total sense now!” things. even if you know the rules of RF it can feel like a very abstract idea, no-less trying to adjust antenna impedances!

  3. I quiver in ecstasy. The thread “Learn machining from an old school metal master” and this one only hours apart! If you start adding threads on beer and one other topic I will leave nameless, I will never be able to leave this web site.

  4. This is awesome. As a ham, I will utilize this. Of course, it should be noted that SWR is not nearly as important as most hams think it is. See Reflections III, CQ Press for full physical description. Still, matching will never hurt, so i choose to do the best I can, within reason.

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  6. Don’t forget that looking at an antenna through the feedline CAN present misleading results. Never forget that SWR is not the only measure of antenna performance. A low SWR only means that the transmitter is “seeing” a reasonably non-reactive load. That is, it is neither capacitive or inductive, it looks like a 50 ohm resistor (a dummy load). Don’t forget that the feedline loss runs both directions and can dramatically affect the SWR reading ! Your transmitter sends power up the feedline (for purposes of this example let’s say it’s 100 watts), and some gets lost going up (let’s say that it’s 20% and 80w gets there). The mistuned antenna reflects some power back down (let’s say it’s 10%, or 8 watts). The 8 watts comes back down, and 20% gets lost, and you see 6.4 watts on the Bird wattmeter. So you see 100 going up and 6.4 coming back, and you think the SWR is a lot better than it really is at the antenna.

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