Last edited 01/04/2003
The MFJ-259B Antenna Analyser V 2.21
A Tale of Two Shoes
It was my shoes that did it. They just kept wearing out as I plodded day after day, month after month.
Each time it was the same story:
There just had to be a better way. And then there was. MFJ introduced their range of antenna analysers.
For me the first instrument was the MFJ-204. This was a fairly simple instrument that would read impedance at a given frequency. Even with this basic information, adjusting aerials became much easier and this sufficed for me for about 8 years.
Then I tried to set up a close coupled array and it became clear that I needed something better to give me a more detailed insight into how the aerial was performing (or not as was the case!). G3SEK reviewed the MFJ-269 in the May 2000 edition of RadCom and gave it the "thumbs up". He compared it to a calibrated network analyser and found that up to 70MHz the agreement was excellent, at 2 metres it was adequate and at 70cm it gave some indication of what was happening. Having no great interest in 70cm I decided to get the MFJ-259B which is exactly the same instrument as the MFJ-269 with the exception of no 70cm coverage and an SO239 connector instead of an N type. These minor changes save over £100.
The instrument is compact and even with the 10 batteries installed, it is quite light enough for use up a tower. The basic readout shows frequency, SWR, R and X. R is the resistive component of the impedance and X is the reactive component. Now if you remember your theory you will recall that positive reactances are inductive and negative reactance is capacitive. The MFJ-259B does not show the sign of the reactance so you can't immediately tell if it is capacitive or inductive. However, remembering that inductive reactance increases with frequency while capacitive reactance falls with frequency, it is pretty easy to decide which you have by simply making the test frequency a little higher and seeing if X increases (inductive) or decreases (capacitive). Don't change the frequency too much however as other effects might well come into play.
For a first test, I took the analyser down the garden and checked my 10 MHz vertical. I have long had a slight problem with this aerial and the match has been sufficiently poor to cut back the power output of my IC737. With the new analyser there, the problem was fixed in 10 seconds! My MFJ-204B had not been up to the task.
As a second task I checked a 21 MHz stub that I had cut for the RSGB 21-28MHz contest in 1999. I had trimmed it with the MFJ-204 and the MFJ-259B showed that it was indeed exactly right. But it also showed that at 28MHz it looked nothing like the open circuit I had expected.
Test three was to check the feeder loss on my 10 MHz vertical. The MFJ-259B showed 1.7dB which sounds about right.
It's really easy to use the MFJ-259B. The manual is clear and detailed. While it does show you how to find whether the reactance is inductive or capacitive, this information was not that obvious.
What's the big deal about resistance and reactance?
If you can determine these parameters reasonably accurately either at one frequency or over several frequencies (for a multi-band aerial for example) you can use a Smith Chart to design a matching network for your aerial either to give a narrow-band or wide-band match. Most often a narrow-band match is what I need so by knowing R and X, I can use software such as the excellent package called TLA (from the ARRL and provided with their antenna handbook) to design the lowest loss (most efficient) aerial matching network. This is engineering, not guess-work - and it can really only be done if you can tell R and X for your aerial.
The Professional Approach
If you do this sort of antenna work for a living, the chances are you'll use a Network Analyser. Of course, in many ways this will be better than an MFJ-259B. For example:
But it will have some disadvantages too:
Resolution and Accuracy
The MFJ-259B has a resolution of 1 Ohm for both resistive and reactive component. This reflects the accuracy of the instrument and so might well be a good idea in many cases. However, if the key requirement for setting up an aerial (say a classic 80m 4 square) is that the reactances of the 4 verticals are the same, it might have been better to have a higher resolution readout. After all, a resolution of 1 Ohm where the reactance under test is of the order of 10 Ohms is a 10 percent uncertainty. I have not done any simulations of critical aerial to see if this resolution might be a problem but my gut feeling is that an extra decimal place might have been handy, accepting that in absolute terms it would not mean much. [Sometime I will do some simulations to see if my gut feeling was right or if I just have indigestion].
Antenna analysers are extremely useful and great value-for-money. If you still struggle with your TX and an SWR bridge, do your shoes a favour and get an analyser - you will not regret it.
I have been doing some construction work today and have found the 259 to be a valuable tool in the shack. I was building a 5 element high pass filter for my FOXX radio. I was able to use the 259 to confirm that the inductors were correct and to identify some dubious looking capacitors that had very faint markings. I tested the filter by terminating it in a 50 Ohm resistor and connecting the 259 to the other end. A hand check that showed the filter did work. I also hit upon another thing you can do with it. If you need a tuned circuit and have, for example a capacitor to hand, you'll need to know what inductance you need for resonance. Just connect the capacitor across the MFJ, set it to your desired frequency and, instead of reading on the capacitance range, switch to inductance - and the value of inductor you need for resonance is displayed directly. Brilliant!