I've found that an important part of building a phased vertical array is being able to make accurate impedance measurements.
I started off using several of the recent group of amateur antenna analyzers. While very convenient, and inexpensive, their accuracy is just not quite what is needed to be a solution. The real problem is accuracy around zero Ohms reactance. Not only is there an accuracy issue, but the sign of the reactance is not indicated, and that adds to the confusion. Still, they are a great value, and it's hard to live without one.
My next trial solution was a traditional impedance bridge, such as the General Radio 916A. The accuracy of these devices is quite good, but they are difficult to use, especially out in the field.
I know that lab-quality impedance analyzers exist, but they are all quite expensive, even on the used market. By expensive, I mean thousands of dollars and up. They would seem to be the ideal solution, if not for their expense.
In the fall of 2003, I became aware of a VNA (Vector Network Analyzer), designed by Paul, N2PK. This design is apparently the result of several generations of designs over several years. The current version offers very good (lab quality) accuracy, and consists of about $200 (USD) in parts. If there is a down side, it is that the parts are not available in traditional kit form, so there is work involved in obtaining the parts and the PC board(s), and then you must construct the VNA using surface mount devices and techniques.
The VNA works in conjunction with a PC computer. I view this as a strength, not a weakness. The use of a computer removes the need for any user interface on the device, and the PC can be used to automate testing sequences and log data. In combination with a laptop computer, you have a device which is almost as portable as an antenna analyzer. Being a general purpose VNA, it can measure many more characteristics than just impedance.
There are several folks who have already constructed VNAs. They have been making accuracy comparisons with various references. All results are very encouraging. Follow the previous links for details.
Although it is certainly possible to purchase parts for a single unit at a time, the cost and inconvenience would be very high. It's very difficult to purchase a single surface mount resistor that might cost 5 cents. All of the suppliers want to sell in quantity, and in return, they offer substantial price breaks. This has led to various group buys, where parts for 10 or more units are purchased at one time. This reduces the cost, and is very efficient.
At this point in time, January of 2004, I've connected with such a group, and am acting as the parts procurer for everything except for the PCB(s), the master oscillator, and some SMA connectors. Parts have been purchased to construct 20 units. Not all of the units have been reserved. If you are interested in purchasing a parts kit from me, please send me email. An email link is included on my home page.
This page will be used to detail my experiences with the VNA project as it unfolds. This will include the construction, testing, and use of the VNA.
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