Saturday, October 30, 2010

MFJ-1621 Portable Antenna

Got myself some additional gear to play with, an MFJ-1621 portable antenna. It was just too tempting...
Well, I don't expect any miracles, in particular with the mixed reviews found on the internet.

The MFJ-1621 portable antenna system
The thing I am curious about in particular, will it be any good for QRSS. Regarding Peter's (DL6NL) great success using a so called MicroVert antenna, I am not expecting a total failure.
Both antennae have a couple of design features in common, a very short radiator, some L/C stuff and a long coaxial feed-line.

The 1621's matching box is said to be good from the 40m band up to the 10m band. Some OM claim the antenna could also be used for 6m and 2m.
I figure, here is where some experimentation could come in. It would be nice to also cover the 80m band, at least partly. The telescopic whip attaches with a 3/8 inch thread also found on CB-antennae. And this could be a start, just use one of those 27MHz whips and see what happens. Additional load inductors the respective threads would be thinkable.

Update (late hours)
Alarmed by some postings (about connections not being soldered), I felt urged to open the tuning box and have a peak. Check it out:

MFJ-1621 tuning box, original state
Well, all connections are soldered, that's good. However, have a look at the flimsy wire gauges used.... When operating on the lower bands, I am expecting a lot of current going through the inductor, returning to the coax-shield via the orange ground-return wire passing by the meter. 
Hence, there is a mod I performed before I even tested the new toy... That is a first actually... have a look:

MFJ-1621 tuning box, first mod
The flimsy orange ground-return wire is replaced by (brown) solid copper mains installation wire. Additionally, I choose to substitute the (flimsy) white whip connector wire by the same stuff. Depending on the performance of this setup, I will (very likely) replace the other r.f. carrying wires with heavier gauges...

As a next mod, I consider to add a coax-connector for the feed-line. I would rather transport three parts (whip/tuner/cable) than just two wherein some cable dangles from the tuner box.

Update (31.10.2010)
Some more findings...
All connection required were soldered, but... still some two fabrication errors to correct. The first mistake was done with the wiring of the band switch, here, the leads to the positions for 18MHz and 24MHz were confused and hence tabbed the coil in the wrong positions.

band switch in the original state
What is to be seen in this photograph? I would like to draw the attention to the tabs shown in the red circle. The green lead tabs the coil for the 18MHz band, the blue-white lead connects the coil to the 24MHz position. In this setup, the 24MHz band would get more inductance than 21Mhz, that does not make any sense, therefore, it only could be a manufacturing mistake.
Have a look at the corrected version.

corrected wiring
Here you can see the corrected wiring, simple exchange of the tabs. I did that soldering with my regular soldering station which was a little weak for all the amount of metal to warm up, the result, not a nice vista, but it works...

I mentioned two manufacturing glitches... that was the first one, the second one was the wrong alignment of the band switch knob. OK, that fix was an easy one!

Conclusion of the bug-fixes: I understand now why some reviews point towards absolute uselessness of the MFJ-1621. My model in the hands of a newcomer would have resulted in a heap of frustration!

Here is a further observation. To understand the following point I am trying to make, some note should be added to the function of the band-switch. This switch shortens tabs if the coil to ground, therefore the full coil will be used for the lowest 40m band position. And now, please have a look at the photo shown below.

band selector switch
I indicated the positions of the respective bands as they are marked on the front plate. You can see 4 positions marked for the 40m band. Actually, only three of the 7MHz positions differ. The one position I labeled 7* is not connected to anything. The 7MHz position next to it, however, is connected to the cold end of the coil and is the common pole of the band switch. Therefore, the switch always shorts the different terminals to the "second" 7MHz position, hence it also shorts the 7* position to it... Well, that does not make a lot of sense. But ok, it does not do any harm either. There may be a use for this terminal, however, I am presently too lazy to think of one.
On air tests: just some short tests. Using the FT817 in its lowest power setting, I could resonate the antenna to all bands. I have to admit that, contrary to the teachings of the user's manual, I did not unroll the coax cable. With the corrected band-switch position, the bands 40m to 17m were spot on. Nothing to complain about. However, for 21MHz and above, it seems advisable to use the antenna set to the next lower band, which will result in a much sharper SWR dip, indicating higher Q, which would help in a resonant antenna. The built-in field-strength meter was slightly deflected even with the transmitter's power settings at the minimum.
With a contest going on, I could hear signals on essentially every band, the reception of the MFJ-1621 is surprisingly good. A clone of such an antenna could possibly be an interesting alternative, also for QRSS-grabbers.