Electronics > RF, Microwave, Ham Radio

Quantity chickens sacrificed for 433MHz antenna?

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--- Quote from: CatalinaWOW on March 18, 2016, 07:05:32 pm ---The SDR could be useful, but is also likely to add to your list of hobbies that require time and effort to understand and put to use.

--- End quote ---
If you are developing something that must transmit wireless signals, a spectrum analyzer is a great time saver.

Spectrum analyzers are very expensive, but a SDR can be a very useful substitute. That's why we are mentioning them  :)

I don't disagree with the usefulness of a spectrum analyzer.  If you have the knowledge, background and time, a SDR is a cheap way to a modest performance spectrum analyzer.

The OP is a beginner with limited understanding.  Remember the purpose is to get a data link going, which if it had gone well would not have sidetracked into antenna design.  Further sidetracking into getting (or heaven forbid writing) appropriate software for the SDR, learning the foibles of its USB interface and antenna connection, and then interpreting spectrum information may be a fascinating further rabbit hole to enter, or it may be a barrier to the original goal. 

My only point was to warn the OP to stop and think about this before proceeding.  It all depends on what kind of learning OP is primarily interested in.  It might have to do with the things on both ends of the data link instead of the data link itself. 

Although a Vector Network Analyser is the best tool for characterising the Impedance of an Antenna or other RF load,it is not essential-----after all,we don't carry a torque wrench around with us for changing a car's spare wheel.
Often,we can get close enough,using,(as in the spare wheel simile),much simpler tools.

Let's recap:-

The OP wrote in the first posting:- "I've attached 17mm copper wires to the antenna hole"
In the ensuing pages we have pointed out the reasons for the classic forms of antenna construction.

We have all assumed that the antenna will be fed from some form of coaxial feeder,so it is an external device,capable of being tested using a VNA,Scalar Network Analyser or whatever.

My suggestion is that chipwitch produce two groundplane antennas as per a "recipe".
If they work satisfactorily as is,no more needs to be done.

A messy,but possible way to "tweak" the antennas is to trim one at a time for maximum signal strength,using your existing transmitter & receiver.

Here,an SDR would be useful.
Replace the receiver with your SDR,set to give a spectrum display,find your carrier,set it a less than maximum level,& trim your receive antenna,watching the signal strength on the display.
Repeat for your Transmit antenna.

There are other ways,such as an RF sweep,used with a directional coupler,either in the form of a sweeper & calibrated detector,a Spectrum Analyser with a Tracking Generator,a standalone Scalar Network Analyser,or various homebrew SWR meters,return loss bridges,etc.---not to leave out the Grid Dip meter.
Some of the latter start to "run out of grunt" at 400 -odd MHz,though.

It's absolutely corect that you don't need a VNA to tune an antenna. However I would say that without one (or as pointed out a scalar network analyser, or an SA with tracking generator with a return loss bridge) it is signficantly harder and time consuming, frequently with a lot more guesswork.

The reason I originally mentioned the USB VNA is that in real terms it's astonishing value for what it does, and not only is it a measuring tool it will allow you to understand the concepts so much more easily. It comes down to how much you value your time. If, like I did, you spend years with an SWR meter fruitlessly trying to match antennas and trying to dovetail the theory with the practice with little correlation, you'll find a VNA an absolute godsend.

Those 3GHz miniVNAs even have a $20 calibration kit. You _have_ to have a cal kit if you want to properly use a VNA.

If you're into understanding and building antennas, a VNA or SA with TG & RLB has more value than an SA on its own, but those minVNAs are cheaper than an SA+TG+RLB. In fact an SA or SDR on its own has little value when designing antennas other than to tell you your antenna is working or not at all: how well it is working needs antenna ranges and/or anechoic chambers, and calibrated antennas and signal sources.

If you're into debugging RF circuits as opposed to antennas, then an SA or SDR plus signal generator or TG is of far more value than a VNA, in fact a VNA has almost no value in this scenario.


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