swr

friis

Senior Member
Hi,
I want to measure swr for my antenna. How can I send continually with Picaxe?
best regards
torben
 

premelec

Senior Member
@friis - the basic issue is deriving SWR values from a device or bridge - which are in some electric form [often a d'arsonval meter] and converting them into voltages that can be easily used by the PICAXE. Sometimes a series resistor can be added and a meter recalibrated to account for its addition and then using the voltage across the added resistor to indicate what the visual meter is reading. Other times the voltage across the visual meter can be used via an OpAmp multiplier to a useful higher value for the PICAXE. These methods do not provide electrical isolation and also RF filtering of the input to the PICAXE may be required to prevent unwanted effects... You haven't specified what unit the SWR values are coming from. There have been articles in the Ham press about making impedance meters but not specifically using PICAXEs that I have seen...
 
Measuring SWR

The most commonly used circuit for measuring forward and reverse voltages on your antenna feeder is probably the Breune bridge. The rectified forward and reverse voltages can be scaled as necessary and digitized by two of the PICAXE inputs. The SWR can be calculated and displayed as required. My home-made automatic antenna tuner uses such a Breune bridge with the tuning (by switching binary stepped inductors and capacitors) controlled by a PICAXE 28X2.

Some more information would be useful, for example frequency and power level.

Richard

Hi,
I want to measure swr for my antenna. How can I send continually with Picaxe?
best regards
torben
 

clockwork

Member
The most commonly used circuit for measuring forward and reverse voltages on your antenna feeder is probably the Breune bridge. The rectified forward and reverse voltages can be scaled as necessary and digitized by two of the PICAXE inputs. The SWR can be calculated and displayed as required. My home-made automatic antenna tuner uses such a Breune bridge with the tuning (by switching binary stepped inductors and capacitors) controlled by a PICAXE 28X2.

Some more information would be useful, for example frequency and power level.

Richard
Have a look at:-

http://www.picaxeforum.co.uk/showthread.php?17710-radio-ham-autotuner-for-magloops&highlight=automatic+tuner

Also there are plenty of SWR measuring circuits and tuning circuits on the internet.

Clockwork G8RIW
 

friis

Senior Member
Hi everybody,
A bit of confusion: I have realised that the output from Dorji 433 Mhz (25 mA) is far too weak for my Maas RX-600 SWR meter - silly.

I think, however, that I have found a way to send continuously for some time:

#Picaxe 14M2
#Terminal 4800

Symbol TXpin = B.5
Symbol TXbaud = n1200
Symbol PreambleLength = 5
Symbol PreambleByte = $55
Symbol SomeByte = $C5
Symbol EOMbyte = $5C
Symbol PostambleByte = $3A

Symbol u = b1
Symbol i = b0
Symbol Sent1 = "1"
Symbol Sent2 = "2"

MAIN: high B.3
pause 500
low B.3

serout TXpin,TXbaud,(PreambleByte,PreambleByte,PreambleByte,PreambleByte,PreambleByte,SomeByte)

do while u < 10000
u = u + 1

do while i < 10000
i = i + 1
serout TXpin,TXbaud,(Sent1,Sent2)
loop
loop
serout TXpin,TXbaud,(EOMbyte,Postamblebyte,Postamblebyte,Postamblebyte,Postamblebyte)

u = 0
i = 0

goto MAIN

but that does not help.

Your proposals contain a lot of new stuff which I will study.

Thanks very much.
torben
 

friis

Senior Member
Hi everybody,
I suppose that I should try and use my MAAS RX-600 SWR meter rather than designing one myself (I am not an electronics engineer)
I suppose that I could use any sine wave generator running at 433 Mhz and outputting at least 0.5 watt to the antenna. But a quick look at the
internet showed none doing that. Is that correct?
Could I amplify my antenna signal sufficiently (0.5 watt)?
torben
 

techElder

Well-known member
torben, do you have any regulations in Denmark on the effective radiated power of your transmitter?

You haven't made it very clear, but it sounds like you are trying to "match" an antenna to a small remote control UHF device?

If so, that is not going to be necessary unless you are trying to boost the power a lot. That probably would break some government regulations.
 

friis

Senior Member
Hi Texasclodhopper,
We have regulations and the Dorji 433 Mhz with the 25 mA is on the limit.
I am trying to match a half wave length dipole antenna (I have three choices) to the transmitter (and the receiver).
But what do you mean by saying that matching the antenna wont be necessary?
I would amplify my antenna signal or use a signal generator only to optimize the antenna - not to boost my operating power.
torben
 

friis

Senior Member
Hi Stan (Swan?),
I think you were the one who introduced me to the Dorji transmitter/receiver pair DRA887/886 some
years ago. I am trying to set up a watering system where 4 satellite sensing systems (Picaxe
14M2) that communicate with a central system (Picaxe 28X) which pumps water. I have had endless
problems, but I have finally arrived at the antennas. There is a house between the satellites and
the central system (distance about 15 m at the most). Quater length mono pole antennas are
unreliable, but a home made dipole seemed to work. But I wanted to optimize/compare a straight wire
dipole, a spiral dipole (smaller than the straight wire dipole) and quarter length monopole with
wire ground plane - just to make sure I had the best, smallest antenna - and out of curiosity.

1. Your article (years ago) said 10/17 dbm output power and TX current 24mA
2. I dont know what a receiver gain antenna is (I am new to electronics - still)
3. The central antenna has to be omnidirectional - satellite antennas can be directional
4. I am looking for reliability
5. The system can handle erroroneous transmissions and receptions
6. I can monitor simultaneously the reception/transmission at the central system and one satellite
system (Picaxe)
7. My RX system does not allow RSSI
8. I dont have a scanner - I could get one (Yaesu FT-60?)
9. My potential helper could wave her arms and use a scanner - I would have to rely on her patience
which is OK

I have stuck with the transmitter/receiver pairs out of stubbernness - wanted to make it work, and
I think it can - at least with a dipole. I may move on to something else once I have succeeded.

In order to overcome the problem of my MAAS RX-600 SWR meter needing at least 0.5 W to move the
needles I thought i could amplify the sending power for antenna testing purposes by using an
amplifier:

https://www.ebay.com/itm/New-35dB1MHz-700Mhz-433Mhz-3-2W-HF-VHF-UHF-FM-RF-Power-Amplifier-Amp-Radio-
Ham-/263284037557

Will that work?

I hope that I have answered your questions!

best regards
torben
 

manuka

Senior Member
Friss - "Torben": Thanks for responses,which by chance arrived while I was tweaking my initial post! Herewith.

I'm a very long time PICAXE Forum member,although these days rarely contribute due to other activities.

PICAXE based wireless data however still grabs my attention - as in your case. Herewith some thoughts that both I (& probably others !) would appreciate your responses to-

* Guess you mean -ah- 25 mW (milliWatts) transmitter POWER rather than 25 mA (milliAmps) CURRENT ? Irrespective such flea power is hardly going to cook your finals with poor SWR (Standing Wave Ratio)!
* EXACTLY what are your control/data needs ?
* EXACTLY what 433 MHz modules are you using (some now are pretty hopeless) ?
* Local UHF RF and physical environment ?
* Does your setup have scope for diverse antenna ?
* Most PICAXE 433 MHz ASK data links are good for only a few 100 m, so do you wish more reliable comms OR better range ??
* Do you realise 433 MHz ISM regulations usually allow only receiver gain antenna ?
* Are you after omnidirectional coverage or directional point to point?
* As 433 MHz setups usually involve control of both the Transmitter (Tx) & nearby receiver (Rx) you can hence probably readily monitor antenna tweaks at the RECEIVER end. Is this your case?
* Does your RX module offer RSSI (Received Strength Signal) info to allow monitoring ? Otherwise -
* Do you have available a 433 MHz scanner of general coverage UHF receiver that'd show signal strength ?
* Do you have a helper who can inform of antenna tweak effects by arm waving, shouting, cell phone or 2 way radio etc?

FWIW - If 433 MHz data transceiver modules (such as the esteemed HC-12) are used, suitable code perhaps could maybe even inform you back at the Tx end of antenna change benefits.

Stan. in New Zealand (Radio ham ZL2APS - since 1967)
You are making it hard for yourself using an SWR meter at this mW level. Exactly which 433 MHz modules are you using ? Some have VERY weak TX (just a mW or so) & quite "deaf" receivers ... Stan.
 

friis

Senior Member
Hi Stan,
I see that my reply to you is at the bottom of page 1.
I have seen a RF Explorer device at about 100 USD. Will I be able to use that for measuring signal
strength from my Dorji 433 Mhz modules? I have been reading about it but there is a lot I dont
understand.
I would expect that as long as the frequency is OK it does not matter wether it is on-off
modulation (as the Dorji modules) or FM or....
torben
 

manuka

Senior Member
Friis: I know RF Explorer(s) well (& prices tend more than US$100)- they may be an overkill. I'd say you only need a simple UHF Field Strength indication,much as you'd check WiFi router coverage etc with an Android app. like WiFi Analyser.

As a starter explore voltages at pin 7 of the RF IC on the DRA887 module ? This is an AGC (Automatic Gain Control) tap point & on similar ASK modules the voltage here varies with RSSI, thus making it a handy monitoring spot when tweaking antenna etc.

Ah- this assumes you're actually using Dorji's modules! From passing comments your 433 MHz devices seem their DRA886/887. Best confirm this first,as Dorji offer many UHF goodies => http://www.dorji.com/products.php?CateId=36

Suggestion: As an RSSI extension it may well be worth grabbing another 433 MHz Rx module & setting it up just for "sniffing" duties in fact. I find such a FSM (field strength meter) EXTREMELY handy for the likes of antenna work, duty cycle, sweet/sour signal mapping & even insights into " is my #$@#$% key fob/doorchime Tx actually going ". See => here and later PT4302 engined SpiritOn RXN3-B module (stocked Aus/NZ as Jaycar's ZW3102) comments at end of => https://www.instructables.com/id/433-MHz-UHF-lost-model-radio-beacon/

Modulation: ASK (Amplitude Shift Keying) receiver modules can be prone to interference,especially in urban regions, & for reliability FM, GFSK or even LoRa are superior (but usually more costly). However a recent ASK Rx offering may have appeal - it'll work with almost any ASK Tx. Check PICAXE Forum discussion =>http://www.picaxeforum.co.uk/showthread.php?30341-Hope-RFM210LCF-433D-A

Simple ASK module battery drain & supply voltages may be an issue too -given today's 3.0-3.2-3.7V Lithium cells it's now wise to avoid portable modules needing 5 Volts & thus multiple supply cells. Terminal corrosion alone can drive you crazy if using the likes of multiple C-Zn/NiMH AAs outdoors !

Extra: It's quite some years (1990s & thus pre PICAXE), since I first delved into simple 433 MHz ASK wireless data. Although individual ASK modules still have educational appeal, far superior single module transceivers are usually now recommended. You may be better going for the likes of the esteemed HC-12 transceivers- see below.

Stan.
 

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friis

Senior Member
Hi Stan,
Thank you for all the information.
Is it not so that SWR measures the efficiency with which energi is delivered to the antenna's
feed point and a Field Strength Meter meter shows how much energi is sent to a specific point
(both ignoring other fields that may be present)? Would'nt I need both?
I have Dorji 886Rx and 887RX. Where can I find the RSSI tap?
I assume that Q1 and Q2 in the setup shown under "here" are amplifyers? Could I put a voltmeter
instead of one of the LED and the Piezo? What is the range of voltages I could expect?
torben
 

friis

Senior Member
Hi Stan,
From your original article (early 2012) I got Pin13,CAGC for the DRA886RX and from another article
I got pin7,CAGC. Is that the rssi tabs? Lower voltage means higher antenna input?
torben
 

manuka

Senior Member
Friis: Yes-that's correct! But please refer my 2012 "tape measure Yagi" Instructable . Although it's antenna & DF (direction finding) slanted, details of the 16 pin RFIC Dorji DRA886RX RSSI tap follow from Step 12.

The RSSI tap technique should be similar if you use Dorji's smaller 8 pin RFIC DRA887RX module.

I again mention that these Dorji modules are now considered "small beer", & for a devoted reliable application you should consider more recent 433 MHz offerings - even transceivers such as the HC-12. These may cost no more (even LESS !), but -as with HopeRF's CMOSTEK's ASK Rx module - offer superior performance with weak signals & interference. They additionally may draw lower current & need friendlier supplies (1.8-3.6V).

FWIW wireless devices are evolving extremely rapidly. Cheap $$ SDR (Software Defined Radio) modules now can link into a smart phone and allow the sort of UHF spectrum monitoring that only $$$$($) professional gear offered a decade ago. At a consumer level 2017 teased us with WiFi/smartphone controlled Xmas lights !

Stan.
 

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friis

Senior Member
Hi Stan,
I will follow your advice. As soon as I have my Dorji modules and field strength meter working I will move on.
Thank you for your help so far.
torben
 

premelec

Senior Member
FWIW I just got my Jan-Feb copy of QEX with an article "Microwave Version of Wideband QRP SWR Meter" by W0PCE - uses 2 Analog Devices ADL5513 logarithmic detectors [forward & reflected [ external Ard...o micro to display SWR] - looks like one of these could be basis for a field strength meter... This unit goes to 4GHz whereas previous author's design went to .5GHz ;-0 Stan's advice is always good!
 

manuka

Senior Member
Premelec: Nice find ! Aside from SWR concerns,the need with 433 MHz setups often relates to optimising the 10/25mW "flea power" transmitter's outgoing radiation punch of course.

Just monitoring the 433 MHz band with a cheap SDR (Software Defined Radio) module, OTG (On The Go) USB connector,Android device & suitable app. may do.

Use such a setup to view the signal strength/quality (& perhaps nearby interference?) of your 433 MHz Tx. Simply visually relate signal changes to tweaking of antenna/feedline/power supply/ data rates/outgoing RF takeoff environment etc. Refer a 433 MHz antenna comparison here

These cheap RTL-SDR dongles have revolutionised UHF band monitoring & signal analysis. Stan.
 

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friis

Senior Member
Hi Manuka,
I have tried to set up DRA887RX as a RSSI using pin 7. However, I am getting a constant 0.65 V from
pin 7 regardless of wether i have tranmitter nearby or not. I have tried to ask Dorji about RSSI
and pin 7, but got no reply. Could it be that another pin should be used? Do you know?
best regards
torben
 

friis

Senior Member
Hi Manuka,
I have ordered the RTL-SDR(R820T RTL 2832U). I hope that will solve my problem.
best regards
torben
 

friis

Senior Member
Hi Stan,
I got my RTL-SDR and got it installed. However, it started to overheat and stopped functioning. I
got a refund and ordered a new one.
I screwed up installing the new one, but I did get it working. There is an enourmous amount of
parameters one can adjust, but I found a (U-Tube) tutorial which helped me get going.
It seems to work fine, but it will take time to get to know all the parameters.
There appears to be quite a bit of transmissions at and close to 433.92 MHz, so I will need a
helper to tell me when my distant transmitter is transmitting so I can identify "my" peak.
torben
 

manuka

Senior Member
Friis: Good to hear of progress. Your "quite a bit of transmissions at and close to 433.92 MHz" may be an understatement, as globally many urban areas have IMMENSE activity around that ISM freq! I live 10km cross harbour from NZ's capital Wellington, & even at that distance the clear RF link means 433.92 MHz "noise" can be considerable- especially at peak times.

This is a significant benefit of the HC-12 modules in fact, as their working freq. can be legally adjusted to 400kHz spaced slots within the ~1.7 MHz wide (433.05-434.79 MHz ) ISM band. Other user noise at 433.400,444.200 or 444.600 MHz may be much less than at 433.800 MHz. Stan.

For testing I'd suggest simply organising PICAXE code to turn your TX on/off every few seconds while you follow progress on the SDR screen. Don't position the TX too far away - just say 10 metres initially may do.

FWIW adding antenna to the TX may be frowned upon by local ISM regs, making the RX end more suitable for antenna & propagation path tweaks. Stan.
 

friis

Senior Member
Hi Stan,
Here are the results from my antenne (10 m away) test with the RTL-SDR dongle and dipole antennas/433.92 MHz:

Helical arms of silver covered copper wire
- strength -8
- SNR 45

Straight arms of plastic covered copper wire/173 mm
- strength -6
- SNR 47

Straight arms of plastic covered copper wire/168 mm
- strength -15
- SNR 41

Monopole of plastic covered copper wire/173 mm
- strength -30
- SNR -

Monopole of plastic covered copper wire with alu.foil under/173 mm
- strength -10
- SNR 6

The first one appears to be the choice.

torben
 

premelec

Senior Member
Looks like second is better - is 'first' chosen because of physical size? What is it's configuration? Thanks...
 

friis

Senior Member
Hi,
The first is chosen because of physical size - and it looks better, and - i think - is good enough.
torben
 

manuka

Senior Member
OK- try moving TX further away now. Any screen & antenna pictures!? Feedline length & type? Adjacent object reflections? Stan.
 

friis

Senior Member
Hi Stan,
I attach a picture of the antennas I have been using. They are all used with vertical orientation.
I thought the two helical antennas should'nt work at all, but they do. I found them in an article
about finding a lost aeroplane - oriented vertically.
Below are the figures for a distance of about 10 and 20 m and with a house with one outer and one
inner (light) wall in between:

SNR strength
antenna 10 m

dipole (173 mm) 41 -3
dipole (168 mm) 44 -4
helical (long) 39 -11
helical (short) 36 -15

antenna 20 m

dipole (173 mm) 40 -11
dipole (168 mm) 33 -25
helical (long) 33 -17
helical (short) 33 -26

Feed line is 50 Ohm coax. No adjacent reflecting objects (about 2 m distance to a metal letter box)

torben
 

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manuka

Senior Member
OK -lots of data coming out of this ! What sort of actual on site project performance are you seeing however?

At UHF a clear LoS (Line of Sight) propagation path can be everything, so perhaps also explore ELEVATING the antenna. Consider somewhat longer feedlines to achieve this, even though they may have losses. Sturdier antenna elements, perhaps with thicker materials, may help too.

Although locating/exploiting a signal take off "sweet spot" is quite legal, I again caution that 433 MHz ISM regs are not usually -ahem- TX antenna friendly...

However, as significant "wasted" high angle radiation comes from dipoles & whips, it'd be worth investigating the likes of 5/8 wavelength or even "SlimJIM" antenna too. ( JIM = J Integrated Match )

I've used SlimJIMs to great effect for VHF AIS (Automatic Identification System) monitoring - they've esteemed low angle omnidirectional radiation/pickup that suits on the ground or marine work. A Yagi would be too directional for many marine applications of course.

Thought: This post now tends away from SWR into SDR monitoring, so perhaps a new topic is justified ? How about "SDR testing 433 MHz antenna" ?! Up to you to begin this ! Stan.
 

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friis

Senior Member
Hi Stan,
When I started out with monopole antennas (a piece of wire 173 mm) I had a lot of transmission
errors. Some (rare) even contained the two bytes which identified them as transactions.
With the RTL-SDR system I could see what was actualy being transmitted on 433.92+-, and it was
quite a lot, which, I think, partly explained the many transmission errors. The RTL-SDR system also
allowed me to see that my dipoles were much better than the monopoles and to see the differences
between the dipoles. I still have to make a choice between them, but it could turn out that they are
all OK with very few transmission errors (< 1 %, which the system can easily handle).
The central systems antenna (which has to be omnidirectional) are at a height of 2 m and the flower
pot (dont laugh) antennas are on the ground, but that does not seem to be a problem.
At the distances I have it appears that I dont have to worry about antenna height or the length of
the feed lines.
I will however try and see how far I can go with the transmitter.

I still have two problems:
I cant see how the helical antennas can work with the direction of the electrical field
perpendicular to the antenna's axis (but they work quite well).
I still think an SWR meter would be useful because it directly tells the effeciency of the
transmission. Do you know of one that can handle the transmission power of 10-30 mw? I bought one,
but it cannot be used for anything below 15 w. Or do you know of the design of one I can make?

Other than that I hope my antenna problems for this system have been solved.

I will make the "SDR testing 433 MHz antenna".

torben
 

manuka

Senior Member
Friis/Torben: OK on your developments & insights - I look forward to a new SDR Forum thread.

Antenna performance of all types can be indeed be unexpected, & helicals (which may be that style for shortening convenience or to act as band traps) are no exception. If it works in your case then use it!

I'm not experienced with SWR techniques at mW power levels,as my past decades with HF ham & Mountain Radio, military & marine VHF, commercial, 11m CB/ UHF PRS gear etc involved transmitters orders of magnitude more powerful than the fleapower 25 milliwatts allowed at 433 MHz ISM.

Heavy duty gear SWR insights can be particularly crucial to both ensure the TX finals are not "cooked" & allow HF antenna/feedlines/impedance matches & tweaking for suitable performance! This can be particularly important to ensure your weak signal gets out when in the field wrestling with random length antenna, dwindling battery power, difficult terrain, demanding conditions & few "skyhooks" !

My past SWR gear tended "needle" (& cross needles) based , but a quick Google shows all manner of both skinflint ideas & digital gear now available - much of the later a likely overkill for simple ISM needs.

REFLECTION-ah-excuse pun: Given the very short ranges & near trivial power needs of 433 MHz transmitters it may be that reliable data reception and practical aspects of the antenna could be more relevant than SWR. If you're trying to save battery power (at either TX or RX) then attention to system "sweet spot" positioning (for near LoS links) & microcontroller "sleep" duty cycles may be far more productive.

Regards - Stan.
 

friis

Senior Member
Hi Stan,
I have posted SDR testing..... - a bit delayed because of comp. problems.
I am using "sleep" to preserve power.
torben
 
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