Quantity chickens sacrificed for 433MHz antenna?

[vk6zgo]

I forgot. There is such thing as a $25 spectrum analyzer.

Have a look at this:

http://www.rtl-sdr.com

Aside from the price and being a dongle, how do they compare (or differ) to VNRs in function?  The Amazon rating is positive.  I'm inclined to buy one tomorrow.  It sounds like these are perfect for someone like me, too many hobbies, too little time and too little money.

To my untrained eye, it looks too good to be true.  .... yet it's only 25 bucks.  Anyone here have experience with them?

The SDR is a Receiver,which with a bit of fiddling can  substitute for a Spectrum Analyser.
What it doesn't have,that a VNA does,is a swept signal which is applied to your antenna,nor does it have any way of looking at the matching of that antenna,much less,a way of displaying the Resistive & Reactive components of  antenna Impedance.

Clever people have worked out ways to provide these functions,& you can probably find websites describing this,but the device linked to is just a receiver,which can be used as a Spectrum Analyser,in a pinch.

[Towger]

Is a Grid Dip meter still a good poor man's alternative to a VNA?  I used to lust after them in the back of the radio mags when I were a lad. In those days I drove the neighbours mad building CB antennas in the back garden.

[borjam]

The RTL-SDR dongles are out of stock at Amazon in the US   poo.  "Expecting" new shipment in a couple weeks.  Or, I can order from their China dealer (supplies low). 

Before I pull the trigger, I did a quick google search and found other brands (some for even less money!).  Since I don't know what I'm looking at to compare them, could someone advise me?  Connectors, protocols, resolution... etc.  What should I be most concerned with.  I think they all use the same chip.  Maybe there's a better one in the same price range?

It's not the same as a VNA, I commented it because someone mentioned a small spectrum analyzer that was on sale some time ago.

That said, it's a simple way to at least do a rough comparation between antennas. And, in the first placem it allows you to verify wether you are actually transmitting or not, very useful to debug problems.

[nfmax]

Here is something that might help you understand the design and properties of aerials:

Circuit theory is only an approximation, a trick to avoid having to solve Maxwell's equations, that works in many common situations but not everywhere. The operation of aerials is one of the situations where it doesn't work.

To describe the propagation of radio waves in free space (no conductors or dielectrics around) Maxwell's equations describe the electric and magnetic fields and the relations between them, and give solutions which are waves that travel at the speed of light. Inside a circuit, where just about everything is a conductor or a dielectric, Maxwell's equations are too complicated to solve and we use the circuit laws: Kirchhoff's laws, Ohm's law etc. These can in principle be derived from Maxwell's equations, but in practice we ignore that and work directly with 'circuit' properties such as current, voltage, resistance, capacitance, inductance & the rest.

An aerial is a strange beast: one side of it interacts with the electric and magnetic fields of the radio waves propagating around it (the world of solving Maxwell's equations), while the other side lives in the world of circuits. At one end of it circuit theory doesn't work, and at the other end Maxwell's equations become too difficult to solve directly. Tricky!

For this reason, once you beyond the realm of cookbook recipes, which have been developed over the years and found to work well, you enter the realm of domain experts, and the specialised (and very expensive) tools they use.

I hope this helps

Maxwell (no relation)

[CatalinaWOW]

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.

[borjam]

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.

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 

[CatalinaWOW]

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. 

[vk6zgo]

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.

[Howardlong]

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.