[hfsig] Antenna analyzer kit

Bruce Raymond bruce at raymondtech.net
Sun Mar 24 22:52:03 EDT 2019

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 

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

supertech0491 via hfsig wrote on 3/24/2019 12:07 PM:
> Reference mention of analyzer project lit, I'd be interested in 
> obtaining  one.
> 73,
> Ed K5KBV
> Sent from my Samsung Galaxy smartphone.
> -------- Original message --------
> From: hfsig-request at lists.tapr.org
> Date: 3/24/19 11:31 AM (GMT-06:00)
> To: hfsig at lists.tapr.org
> Subject: hfsig Digest, Vol 40, Issue 4
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> Message: 1
> Date: Sun, 24 Mar 2019 11:29:56 -0500
> From: supertech0491 <supertech0491 at yahoo.com>
> To: hfsig at lists.tapr.org
> Subject: Re: [hfsig] hfsig Digest, Vol 40, Issue 3
> Message-ID:
> <mailman.36.1553445075.809.hfsig_lists.tapr.org at lists.tapr.org>
> Content-Type: text/plain; charset="utf-8"
> I want one of the ant analyzer kits. If you come up with the kit, I'll 
> provide addy, K5KBV, EDSent from my Samsung Galaxy smartphone.
> -------- Original message --------From: hfsig-request at lists.tapr.org 
> Date: 3/24/19  11:00 AM  (GMT-06:00) To: hfsig at lists.tapr.org Subject: 
> hfsig Digest, Vol 40, Issue 3 Send hfsig mailing list submissions to 
> hfsig at lists.tapr.orgTo subscribe or unsubscribe via the World Wide 
> Web, visit 
> http://lists.tapr.org/mailman/listinfo/hfsig_lists.tapr.orgor, via 
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> line so it is more specificthan "Re: Contents of hfsig 
> digest..."Today's Topics:?? 1. Re: 20m WSPR shield for Raspberry Pi 
> (John, 
> W9DDD)----------------------------------------------------------------------Message: 
> 1Date: Sat, 23 Mar 2019 14:29:53 -0500From: "John, W9DDD" 
> <w9ddd at tapr.org>To: hfsig at lists.tapr.orgSubject: Re: [hfsig] 20m WSPR 
> shield for Raspberry PiMessage-ID: 
> <dcfdb486-6eca-e59e-d85a-6dead1b8e325 at tapr.org>Content-Type: 
> text/plain; charset=windows-1252; format=flowedI want one of those 
> antenna analyzer kits.John, W9DDDOn 3/23/2019 2:23 PM, Bruce Raymond 
> wrote:> Hi Keith,> > You've obviously picked the ultimate in terms of 
> getting rid of hum - > using a battery (I didn't read the original 
> post carefully enough). > You've hit on all of the fixes I can think 
> of.> > 120 Hz implies full wave rectification of 60 Hz. Unfortunately, 
> 60 Hz is > all around us, so that doesn't help much. Another 
> relatively obvious > thing is to question the test equipment, although 
> that's a real long > shot. I don't see how you get mixing products 
> from a spec an or a scope. > I suppose an easy test for that would be 
> to look at a known clean > signal, like from a signal generator, and 
> look for mixing products. This > whole direction seems like a real 
> long shot.> > You've got more power coming out of the transmitter than 
> we would > expect. That probably means that the MOSFET bias voltage is 
> fairly high > relative to its threshold voltage, so the bias current 
> and gain are > high. It's possible that you're generating much 
> stronger harmonic > components from the MOSFET than you otherwise 
> would, raising the > harmonic content of the output.> > OTOH, 30 dB 
> down means ~3% of the amplitude relative to the carrier. I > doubt 
> this would have any impact on a WSPR receiver's ability to decode > 
> the signal.> > > I originally put together a 30m WSPR transmitter kit. 
> I sent ~45 of the > kits to John/TAPR. These are sold out. I 
> redesigned the unit to use > surface mount parts and had 200 units 
> assembled and tested, and > delivered to John. There should be plenty 
> of these units available.> > 73 Bruce> > P.S. I'm working on an idea 
> for an automatic antenna analyzer kit for > HF. It will scan a range 
> of frequencies and find the resonant point and > VSWR at resonance 
> (actually min VSWR, not necessarily resonance), and > 2:1 VSWR 
> bandwidth. This will be a kit with no surface mount parts and > no 
> case, but will have mounting holes so you can mount it in your choice 
> > of cases. It will probably sell in the ~70 range. Does this sound 
> like a > worthwhile project?> > > > Keith Wilson wrote on 3/23/2019 
> 1:02 PM:>> Hi Bruce, and thanks for the generous response!>>>> My 
> power measurement was made with a good 50 ohm load so maybe I have >> 
> a nice hot MOSFET!>>>> As mentioned in my previous email, the testing 
> was with a USB power >> bank battery, and I still have the 120 Hz 
> apparent mixing products.? I >> tried a separate antenna (my SOTA 
> antenna) so I was not attached to >> the station ground.? (My station 
> ground is bonded to the household >> power ground) So I shouldn't have 
> any way for 60 Hz power to get into >> the Pi or USB battery.? Still 
> puzzled by the presence of these >> apparent mixing products.? Because 
> they are down 30 dB, I probably >> won't get double decodes by 
> receiving stations.>>>> Last question, TAPR is out of the 30 m 
> boards.? Are they still available?>>>> 73,>> Keith - KE4TH>>>> On Fri, 
> Mar 22, 2019 at 9:14 PM Bruce Raymond <bruce at raymondtech.net >> 
> <mailto:bruce at raymondtech.net>> wrote:>>>>???? Hi Keith,>>>>???? I 
> have to agree with Bryan (well, I suppose I don't *really* have>>???? 
> to agree with him, I just want to :-). The 20m transmitter is>>???? 
> Zoltan's design, but very similar to my 30m transmitter. The 
> final>>???? is an MMBF170 powered by 5 volts.>>>>???? 1. The power 
> output is approximated by the formula?? P = V^2/2*RL.>>???? The power 
> supply is roughly 5 volts and assuming a 50 ohm load>>???? (RL), P = 
> 5^2/(2 * 50) = 0.25 watts.>>>>???? It's reasonable to expect some 
> losses and the safest way to list>>???? the output power is to say 
> you'll get at least 200 mW. Also, if>>???? the supply voltage is 
> higher than 5 volts then you'll get more>>???? power. It's unlikely 
> that it would be *that* much higher; it would>>???? take 6? volts to 
> give 360 mW. Another possibility is that your>>???? antenna impedance 
> is less than 50 ohms. If your antenna impedance>>???? is, say, 35 
> ohms, then P = 5^2/(2 * 35) = 360 mW. The last (and>>???? most 
> probable) thought is that the MOSFET in your transmitter is>>???? 
> hotter than typical and gets driven harder, producing more 
> output.>>???? I've played with this on the 30m transmitter and have 
> gotten power>>???? outputs in this range by biasing the MOSFET on 
> more. The threshold>>???? voltage for a MMBF170 MOSFET is between 0.8 
> and 3.0 volts with 2.1>>???? volts being a typical value. The 20m 
> transmitter has a voltage>>???? divider putting 2.3 volts on the gate. 
> If your MOSFET is fairly>>???? hot then it would be biased on more and 
> likely put out more>>???? output. The end result is *yes* the output 
> is real. => Watch for>>???? the MOSFET getting hot. If it does, you 
> might want to add a heat>>???? sink or change the gate bias resistor 
> (R2) from 1.2K to something>>???? larger, say 1.5K. <=>>>>???? 2. 
> Power supplies - in the words of Socrates, suffering an>>???? learning 
> are two names for the same experience (I don't know that>>???? 
> Socrates actually said that, but I like to say he did). I have>>???? 
> learned through hard experience that inadequate power supplies>>???? 
> cause a whole bunch of problems, and they're usually very>>???? 
> difficult to troubleshoot because the problems are either>>???? 
> intermittent or just not something I'd normally suspect of a 
> power>>???? supply. The power supplies normally used for the Raspberry 
> Pi are>>???? usually marginally adequate. I'm very impressed with the 
> job the>>???? designers of the Pi did, but they cheaped out on the 
> power supply>>???? filter on the board (electrolytic 
> capacitor).>>>>???? Now we compound the problem with trying to run a 
> transmitter off>>???? of the same power supply in addition to running 
> the Pi. This>>???? doesn't help things. In the beginning I bought a 
> bunch of cheap>>???? 5V/2A power supplies from China that worked with 
> my Pi/30m>>???? transmitters. I had a bunch of weird problems, such as 
> the>>???? software getting corrupted during normal operation. At first 
> I>>???? thought the problem was cheap SD cards or some problem with 
> the>>???? operating system/software. I now believe the problem was 
> power>>???? supply glitches causing the Pi to get confused and do bad 
> stuff. I>>???? switched to bigger power supplies and my problems 
> disappeared.>>>>???? My recommendation is to get a 5V/3A power supply 
> and make your>>???? measurements again. 120 Hz sidebands sounds like 
> AC bleeding>>???? through the power supply, even if it seems that the 
> power coming>>???? off the supply is clean. It could also be some sort 
> of interaction>>???? between your antenna ground and your power supply 
> ground. You>>???? might try a different power supply and/or an 
> isolation transformer>>???? for a test. This might be similar to hum 
> problems direct>>???? conversion receivers have that are associated 
> with grounding.>>>>???? 73 Bruce Raymond/ND8I>>>>>>>>???? Bryan 
> Corkran wrote on 3/22/2019 4:22 PM:>>>???? I had a lot of trouble with 
> power, in the end I bought the>>>???? ?official? 2.5 amp adapter and 
> had no trouble after that.>>>???? Keith is right the shield is 
> designed for the V1 board hence the>>>???? little slot in the middle 
> for the display port. I had problems>>>???? with the shield fouling on 
> the heat sink I?d added on a 3b board>>>???? so I used a GPIO extender 
> to raise it a small amount.>>>>>>???? Bryan, VK3KEZ>>>>>>???? On 23 
> Mar 2019, at 5:36 am, Keith Wilson>>>???? <keith.wilson.pcs at gmail.com 
> <mailto:keith.wilson.pcs at gmail.com>>>>>???? wrote:>>>>>>>???? I have 
> the 20m WSPR shield working on a new Raspberry Pi 3 B+. >>>>???? I see 
> apparent mixing products in the output, 120 Hz away from>>>>???? 
> fundamental, when using a USB power bank to power the Pi.? 
> Since>>>>???? these are not coming from a switching power supply, 
> where are>>>>???? they coming from?? These products start at about 30 
> dB below the>>>>???? fundamental.>>>>>>>>???? Also, with a scope I 
> measure the voltage output at 12V peak to>>>>???? peak into quality 50 
> ohm dummy load.? This is 0.36W, higher than>>>>???? the 20dBm (0.10W) 
> specified.? Is this too good to be true?>>>>>>>>???? Note the shield 
> was not designed for the Raspberry Pi 3 B+ so it>>>>???? can't be 
> fully inserted on the 40 pin GPIO plug, but seems>>>>???? stable 
> enough partially inserted.? Getting WSPR reports from>>>>???? across 
> the USA and occasional overseas reports too.>>>>>>>>???? Keith - 
> KE4TH>>>>>>>>???? 
> <https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail&utm_term=icon>>>>>???? 
> Virus-free. www.avast.com>>>>???? 
> <https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail&utm_term=link>>>>>>>>>>>>>???? 
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