MFJ-259B Antenna Analyzer

 


MFJ-259B Antenna Analyzer

If you intend to produce many homebrew antennas to your own designs or adapt the designs of others to better meet your needs then one of these is pretty much essential. With just an SWR meter and a rig then tweaking the SWR on a commercial mobile or trimming a wire dipole for resonance are fairly trivial but tackling the tuning of something more complex like a loaded element that is way off resonance on the desired frequency will very soon have even a saint cursing.

As an example I bought a commercial multiband HF vertical – a Sandpiper V5. This is essentially a 3m long end fed vertical with a number of loaded whips connected to the top (one for each band). The guideline length for one of the whips was way off and I had to trim down the adjustable section to bring it to resonance. Without the analyzer I really would have been working in the dark as the VSWR at both ends of the band was around 10:1 with no clue which way to go. Checking the resonant frequency, adjusting by 10cm (iirc) and noting the new resonance allowed me to calculate the length required. Naturally I didn’t trim it right down to my calculated value immediately in case of error and did go there in a couple of steps.

This analyzer has naturally been my guide on every antenna I’ve made or tuned since buying it. I won’t go so far as to say that I couldn’t have produced or tuned those antennas without it but the task would have been a whole lot harder and I wouldn’t have been so confident that things were correct – this is especially so for the 137MHz weather satellite antennas where a rig/swr meter simply couldn’t be used.



Unusual usage
I have not seen mention of one use I have put this unit to – quite possibly because it isn’t a legitimate usage but I really can’t see why not. The efficiency of an end fed vertical is largely determined by the quality of the radial system employed…

A full electrical quarter wave has a radiation resistance of around 35 ohms. Shorter radiators have a lower radiation resistance – sometimes as low as a few ohms in the case of HF whips shortened to the point they can be used on a vehicle. Yet antennas such as these can usually be matched to the 50 ohm impedance required by the transistor output stage of a modern HF transceiver so how can this be? Well a large proportion of the additional ohms needed are made up of lossy loading coils and earth systems with the rest from off-resonant reactance.

After laying a rudimentary ground system for my Sandpiper V and tuning it up I found a resonant point X (reactance) = 0 within or close to each band and noted the R (resistance) indicated by the MFJ-259B. By rechecking this after adding radials I could infer how much those extra radials had reduced the ground losses from the reduced R. I retuned the antenna after completing the ground system.

To get a proper measurement of the ground loss would require a full quarterwave radiator at the frequency concerned then simply measuring R and subtracting 35 should give a pretty good estimate of the ground loss. I didn’t take it that far nor did I continue past 16 radials and certainly not to the 100+ reckoned by some to be the point where diminishing returns make adding more a pointless exercise.

Points worth noting
The unit is electrically fragile and users should read the MFJ 259B User Guide carefully before use to avoid destroying the broad band voltage detector diodes it (and all cheap analyzers) use.
For convenience and most economical use take advantage of the ability of the unit to use NiCads – these can be recharged in the unit which has a built in charger.



  2 Responses to “MFJ-259B Antenna Analyzer”

  1. The 259b is a very useful unit, worth every penny. Had mine for about 1yr, Only problem was the solder joint at the base of the 239 socket and the board, there is a u shaped in the board which the base of the 239 solder pin fits in but there is a gap which is bridged with solder and comes off with any movement of the 239 socket. I improved the joint with wire from the socket pin to the board and problem solved. The 259b saves hours when setting up any aerials, tuners baluns etc.

  2. the 259b is a very good bit of kit for antenna tuners ,could the 259b be modded to work at 433mgs as does the 269? regards ken,m3zkb,rafars 4680

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