Checking for self resonance of yagi antenna with TinySA, Possible? - Page 1
Forum section: electronics / rf-microwave

author: CaptDon
author_rank: Super Contributor
timestamp: April 06, 2025, 02:58:15 pm
content: I want to find the self resonance point of a UHF yagi antenna. My signal generator doesn't go high enough in frequency for the test. Is there a clever way to use my TinySA to look for self resonance or at least a gain peak near the frequency of interest? It is for receiving only and I have found that 'resonant' antennas seem to work best for reception even if the transmit V.S.W.R. isn't ideal. Thanks for any ideas!!

author: TimFox
author_rank: Super Contributor
timestamp: April 06, 2025, 04:35:22 pm
content: What frequency range needs to be covered?

author: CaptDon
author_rank: Super Contributor
timestamp: April 06, 2025, 08:46:15 pm
content: Somewhere around 545 to around 565 MHz. A portion of the packed UHF television band. NBC, ABC and PBS are solid copy where I live. Those stations are directional with a pattern like a butterfly. I am in one of their major lobes. They are around 500KW E.R.P. The CBS outlet which is marginal for me is 96KW E.R.P. and omni directional but located in the same place as the other stations. I have a good Yagi, one of the better available and I have an antenna mounted amplifier. With no leaves on the trees I see around -75dBm but in the summer the signal drops to around -85dBm and my T.V. pixelates or won't lock on at all. Those signals are measured directly at the antenna prior to the amplifier. My amplifier claims 10 to 15dB of gain and I believe it. I think my amplifier may be suffering desensing or IMD because I live very close to cell towers and also near the city courthouse antenna farm and a rail yard. I say that because the weak signal is 3 out of 5 bars and suddenly drops to zero bars. I am trying anything I can to get more gain (additional antenna height is not possible). My amplifier has no rejection filters on it. I see there is an amplifier on the market that is supposed to have separate VHF / UHF internal amplifier paths with A.L.C. and 5G filtering. Maybe worth an amplifier swap? I know if a signal is poor to begin with an amplifier often makes it worse but -85dBm seems strong enough to work with and I am hoping to 'tweak' the signal prior to amplifying.

author: LM21
author_rank: Regular Contributor
timestamp: April 07, 2025, 07:19:42 pm
content: Two TinySAs? Other at signal generator mode.A cheap signal generator from Ebay, or whatever, connected as source, and TInySa as receiver.But I would get a NanoVna. There are several models, some go to 3GHz at reasonable price. When it works at 3GHz+ it surely works at 400MHz. (https://groups.io/g/NanoVNAV2/https://nanorfe.com/nanovna-v2.html)?

author: DimitriP
author_rank: Super Contributor
timestamp: April 07, 2025, 10:21:04 pm
content: Point it towards noisy LED lights ?

author: radiolistener
author_rank: Super Contributor
timestamp: April 08, 2025, 01:27:45 am
content: you can make or buy VSWR bridge for that

author: antenna
author_rank: Frequent Contributor
timestamp: April 08, 2025, 08:24:35 am
content: Does your signal generator have a square wave option? Square waves are just odd harmonic sine waves added together, so if your sig gen goes up to 60MHz, setting it to 50.45 would put some energy on 555MHz. Not much power, but enough to see on the tinySA as it sweeps the antennas feed point with the antenna aimed at the sig gen clip leads. Obviously, the higher your sign gen goes (lower harmonic you need to use) will provide better the results, but I've used my 60MHz sig gen to make UHF before.

author: CaptDon
author_rank: Super Contributor
timestamp: April 08, 2025, 02:03:06 pm
content: My signal generator is a Wavetek 3000. They spec 1MHz to 512MHZ. Mine will do 400KHz to around 525MHz before unlock occurs. I never looked at what it would do above unlock. Maybe today I'll put it on the scope (Tek TDS644B which reliably displays signals to 1GHz) and see if it is usable at 550MHz in an unlock condition. Someone suggested a VSWR bridge. As I mentioned in the previous post "often a resonant antenna works better even though the VSWR is not ideal. Case in point, the old MFJ SuperLoop. At good VSWR they didn't work worth a shit!!!! You tuned for peak receive noise and accepted the VSWR whatever it was. Used that way I made trans-Atlantic contacts with 5/9 reports on both ends on 20 meter SSB.

author: antenna
author_rank: Frequent Contributor
timestamp: April 08, 2025, 08:30:09 pm
content: Quote from: CaptDon on April 08, 2025, 02:03:06 pmI did not say to run it in an unlock condition, DO NOT TAKE THE COVER OFF THAT GEN, I was suggesting running it at a sub-harmonic that is within the range of the generator. However, now that I know what generator you have, I can say that this will not work as that model does not have a square wave output.The next option is to make a frequency doubler or a clipping circuit for the sig gen output to get some higher frequency harmonic content in the signal. Maybe a shottky diode in series with a resistor across the sig gen output to clip one side of the waveform. Asymmetric distortion produces even harmonics, so running at half the yagi frequency into such a clipping circuit would generate some energy on the yagi frequency.

author: tszaboo
author_rank: Super Contributor
timestamp: April 08, 2025, 08:34:44 pm
content: Don't you have a VNA to measure that? Get one, especially for UHF VNAs like the NanoVNA are dirt cheap.

author: CaptDon
author_rank: Super Contributor
timestamp: April 08, 2025, 09:36:36 pm
content: Antenna, Over the last year I have learned more about that Wavetek 3000 than most people know. Mine is actually wearing the Motorola brand name but it is a Wavetek 3000 for sure. I bought it broken and repaired nearly every module!! The power supply went ape shit and destroyed many of the opamps. It works great now but I suspect more failures may be occurring. A couple of tantalums also shorted due to excess voltage. The 5vdc rail was fine. The +/- rails went crazy. Maybe I'll buy a VNA to go with my TinySA. Big word of warning on the 3000 unit...NEVER power up the unit with the module securing screws missing!!! It has a very stupid design flaw...The modules get their ground through the screw (and SMA cables) and running a module ungrounded will kill it!!!