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.

The Blibber

Triggered by the comment Guido wrote in the KnightsQRSS-blog and with some experience in QSK-QRSS (see my ealier blabla on the QRSS-ified RockMite) combined with Colin's broad-A1A-QRSS the following idea evolved.

What about a "fast QSK", some would call it TDM, switching between RX and TX constantly in a comparably short period. What about going TX for 1/3sec and RX for 2/3sec. Obviously, some SNR will be lost for the RX and the average transmitted power will be just 30%. Additionally, the transmitted power would create sidesband, which, in this particular case, may even be wanted in order to add signature to the signal. In a regular receiver one would simply hear "blibb blibb blibb".

Speed: Since there will be only one blibb created per second, there would be a reason to slow down the message. QRSS10 would probably be the fastest one could go. I calculated that my relatively long ident (not full call!) would take about 6min 30sec. That would nicely fit into a 10min frame for averaging.

Averaging, speaking of it, could help for the receiving too. The trick could be to divide the 1sec period into three slots, in which two are reserved for receiving. Now, instead of transmitting constantly in one slot, the TX-slot could glide for every 10min cycle. Averaging the received spectra of three cycles would display the full 10min cycle.

Downside: I believe that not only the TXed signal will get some side-bands to it, but also the received signal will suffer from the same. However, it may be desirable to have those side-bands transmitted, for reception, those side-bands could render the spectra really messy.

For sure, there always will be the option of slowing the whole thing down to some WSPR-ish period, avoiding the side-bands.

20m & 40m Dual Band Digimode-TRX

The 2048kHz crystal appearently got some potential, more than I initially thought.
In my previous entry, I motivated digimode superhets for 40m and 20m. The 40m version would be build around a 5.000MHz filter, for the 20m version, a 10MHz filter will do the trick. As you may recall, the idea for 20m was to use a subharmonic mixer, as to have an effective local oscillator frequency of 4096kHz. Well, the same could be done for the BFO. The 40m version already employs a 5MHz BFO, which could be easily used for the 20m version, provided the product-detector is subharmonic too.

And here is the idea for the RX part:

• one switchable front-end
• one (super-)VXO for 2048kHz, maybe using an additional penned down one, lets say for 2035kHz
• one BFO for 5000kHz, slightly pullable, as usual
• two mixers, one for regular mixing, one for subharmonic mixing
• two IF circuits having crystal (ladder) filters, one for 5.000MHz, one for 10.000MHz
• two product-detectors, one for regular mixing, one for subharmonic mixing
• one audio stage

At first sight, this looks like an expensive solution, however, I believe it actually is a cheap one, since crystals for 5.000 and 10.000MHz cost next to nothing. For that reason, one could consider building individual transmit filters too. So the TX part only need the following additional building blocks:

• one input audio stage
• two modulators, one for regular mixing, one for subharmonic mixing
• two IF circuits having crystal (ladder) filters, one for 5.000MHz, one for 10.000MHz
• two converter mixers, one for regular mixing, one for subharmonic mixing
• one switchable linear amplifier

I would stronlgy recommend to use two crystals for the LO. One on the original 2048kHz, which allows for WSPR on 20m and DATA on 40m, and one that is penned down to about 2035kHz, that one will be good for WSPR on 40m and PSK31 on 20m.

2048kHz XTAL

Haven't got the luck to obtain any of the before mentioned 2030kHz crystals? Well, there is hope ;-)
What about the 2048kHz standard crystal? This one should be widely available.
Let's see what we can do with this one. First of all, such crystals can be penned down, 18kHz should be doable. But what more can we do?

600m I/Q-SDR
For the typical SDR, one could simply feed this signal into two Flip Flops, resulting in an SDR LO of 512kHz. With a minimum sampling rate of 24kbps, a range from 500kHz to 524kHz would be covered. This is in particular interesting since 518kHz, the international NAVTEX frequency falls into this range.

2200m I/Q-SDR
Taking one of the two 512kHz signal and feed it into two further Flip Flops would provide an SDR-LO of 128kHz, which would provide a frequency range of 116kHz to 140kHz, covering the 136kHz amateur radio band.

40m Digi-Mode Superhet
As non SDR use of the crystal, a more classical approach is in reach: a 2048kHz (super-)VXO and a 5000kHz crystal filter coincides nicely with the 40m digital mode range. The 2048kHz crystal could additionally penned down (see earlier post about xtal-penning) to cover the 40m WSPR and PSK frequencies.

20m Digi-Mode Superhet
In the 20m band, the WSPR frequency is easily covered. The trick here, a 2048kHz (super-)VXO as LO for a subharmonic mixer, which per se doubles the LO-frequency. Hence, the effective LO-frequency would be 4096kHz. In such a concept, it would be somewhat obvious to use a 10MHz crystal filter.

10m CW Superhet
Similar to the 20m concept, a subharmonic mixer would be required. As intermediate frequency, 24.000MHz would be obvious. However, watch out for the correct crystals for the filter! Most crystal will be overtone crystals, for a cheap filter we would need fundamental frequency crystals, and yes, they do exist for 24.000MHz.

40m LSB Superhet
Here is a simple one. The sum of 2048kHz and 5068.8kHz results in 7116.8kHz. I would propose to build a full-lattice filter with two original and two penned down 2030kHz crystals. As LO a super-VXO using 5068.8kHz crystals would an obvious choice. Additional options would be a 5120kHz VXO (getting us to 7168kHz), a 9216kHz VXO (9216-2048=7168), or, for some of us, a 5200kHz VXO (7248kHz).

80m LSB Superhet
This could be a tricky one. I would, once again, propose to build a full-lattice filter with two original and two penned down 2030kHz crystals. A conceivable could employ a 5.74MHz ceramic resonator. Subtractive mixing would provide a range, depending how far one pulls the VFO, of a couple of 10kHz about 3692kHz.

4096er Grabber Receiver
You may remember that my 30m grabber receiver employs a subharmonic mixer. Well, the exact same can be done for the 4096er hf-beacons. However, those beacons spread a little bit more than the 100Hz wide 30m QRSS segment, therefore, I would skip the crystal filter. This would have the advantage of also showing beacons below the 4.096MHz nominal frequency in a good old fashion DSB way.

4096er I/Q-SDR Grabber Receiver
One of the most popular entries on this blog is concerned a subharmonic I/Q-SDR in which the 90 degree I/Q phase-shift is done on half the frequency and hence amounts to 45 degrees on the subharmonic local oscillator. The exact same could be done for the 4096er beacon range (click here for more info) using a 2.048MHz crystal and some RC/RR network as shown in the subharmonic SDR article.


There may be more uses of this crystal, feel free to add comments with additional ideas!

2030kHz XTAL

Browsing one of the regional electronic (online) shops, I found a 2030kHz crystal, which would be perfect for a couple of projects. The order seems a little odd, however, the order reflects the difficulty of the different projects, the further down, the more difficult to realize.

600m I/Q-SDR
For the typical SDR, one could simply feed this signal into two Flip Flops, resulting in an SDR LO of 507.5kHz.
With a minimum sampling rate of 24kbps, a range from 495.5kHz to 519.5kHz would be covered. This is in particular interesting since 518kHz, the international NAVTEX frequency falls into this range.

2200m I/Q-SDR
Taking one of the two 507.5kHz signal and feed it into two further Flip Flops would provide an SDR-LO of 126.875kHz, which would provide a frequency range of 114.875kHz to 138.875kHz, covering the 136kHz amateur radio band.

40m QRP Superhet
As non SDR use of the crystal, a more classical approach is in reach: a 2030kHz (super-)VXO and a 5000kHz crystal filter coincides nicely with the 40m QRP frequency. The 2030kHz crystal could additionally penned down (see earlier post about xtal-penning) to cover lower frequencies in the 40m CW section.

20m QRP Superhet
Also the 20m QRP frequency is easily covered. The trick here, a 2030kHz (super-)VXO as LO for a subharmonic mixer, which per se doubles the LO-frequency. Hence, the effective LO-frequency would be 4060kHz. In such a concept, it would be somewhat obvious to use a 10MHz crystal filter.

10m QRP Superhet
Similar to the 20m concept, a subharmonic mixer would be required. As intermediate frequency, 24.000MHz would be obvious. However, watch out for the correct crystals for the filter! Most crystal will be overtone crystals, for a cheap filter we would need fundamental frequency crystals, and yes, they do exist for 24.000MHz.


40m LSB Superhet
Here is a simple one. The sum of 2030kHz and 5068.8kHz results in 7098.8kHz. I would propose to build a full-lattice filter with two original and two penned down 2030kHz crystals. As LO a super-VXO using 5068.8kHz crystals would an obvious choice. Additional options would be a 5120kHz VXO (getting us to 7150kHz), a 9216kHz VXO (9216-2030=7186), or, for some of us, a 5200kHz VXO (7230kHz).


80m LSB Superhet
This could be a tricky one. I would, once again, propose to build a full-lattice filter with two original and two penned down 2030kHz crystals. A conceivable could employ a 5.74MHz ceramic resonator. Subtractive mixing would provide a range, depending how far one pulls the VFO, of a couple of 10kHz about 3710kHz.


There may be more uses of this crystal, feel free to add comments with additional ideas!

The Broompole

You may have seen the previous post, the proposal of faking a Trans World Antenna. Mechanically such a clone would be a challenge, in particular getting good conductivity over the hinges. Maybe I will take on that challenge some other time.

However, what about back to the idea of using the broomsticks for a loaded shortened dipole. But, how get hold the stuff together? BTW, those broomsticks are 1.3m long and have a diameter of 22mm.

This is what I came up with, when searching the hardware store for a solution. The handle snug fits into a 32mm PVC plumbing bit, as shown below.

The idea is now to connect two of those bit by some pipe, such to create space for mounting means and the loading box. Oh, I forgot, I intend to build a center loading box, similar to the one in the Trans World Antennae. OK, back to what I was saying, you can see that the handles of the sticks are equipped with an eye. This comes along quite handy, since the eye more or less aligns with the T-piece's third port. The plan is to rope both broomsticks together which would enable a solid but still easily collapsible setup.

Note, those sticks could be just ideal for a 6m dipole.

Remains to put it all together....

The Broomtenna

The local hardware store offers broomsticks made of aluminum for a price of next to nothing. Some months ago, I picked two of those up, the plan was to build a loaded dipole of sort... I never really got to use them for anything interesting yet, until today, an idea came along.

This could be a blue-print for things to come:
http://transworldantennas.com/
Looking a the TW-antennae, this is what I can see: a centre loaded short vertical dipole with end-capacitors. The company offers manuals for download, I could not resist doing so ;-) The only difference between the various models seems the centre loading box, the mechanical structure appears common to all models.

And here the broomstick come into the game. I figure, a similar structure can be build using 6 of those above mentioned aluminum broomsticks. Electrical insulation, like for the centre piece of the connection to a stand, can easily be done using PVC-plumbing parts.
For ease of manufacturing, I would probably go for a modular setup inside the loading box, meaning, exchangable sets of coils for different frequencies. The box holding the loading network will therefore be picket slightly oversized with a skrew-on lid. I recall to have seen boxed for outdoor electrical installations having such properties.

Not sure when to start with this project, I will however post pictures, as soon as there is something to show.

New Type of OTHR?

Today, for the first time ever, I see this sort of signal in 30m-spectrum-grabbers worldwide.

 

To me, this looks like some sort of OTHR, I may be wrong here... Anyone an idea?

Rhombic DCTL

Winter and storms arriving, it was time to do something about the wobbly design of the grabber's antenna.

Having had very good results with PE plumbing parts for my weather proof 600m frame antenna, I went to the hardware store today to buy some other material.
Contrary to the 600m stuff, which uses very heavy speaker cable, the DCTL uses lightweight TV twin line and only turn of it. This allows for flimsy 5/8" PE electrical installation tubing. Have a look at the b.o.m.:

My DCTL is 327cm long. An elbow is good for 8.5cm. In order to compensate for the shorter path through the terminal box, the two tubes connected to said box are 2cm longer than the other two.
Consequently, I cut two tubes to the length of 73cm and two to the length of 75cm.

Since today, 1903z, the DCTL is back online, in "tip up" rhombic configuration.

BTW, another reason for selecting this configuration was the wish to lift up the aerial by means of a pole, to improve ground clearance.

Note, the tubes are held together by Duck-Tape. Plumbing glue could be used, I guess, however, tape does not create such a stink ;-)

WSPR 2.1 - a possible solution for 600m

Background

Joe, K1JT, recently made WSPR 2.1 available which now employs I/Q-SDR. Primarily it seems to be intended for the SoftRock RXTX Ensemble.
I figure, with the correct tweek, it could be made to operate on the 600m-WSPR range as well. Not sure about the precise working of the I/Q-SDR option of Joe's software, so 2 scenarios are possible:

1. An ideal case would be that the software employs its own SDR-LO, meaning that it would be sufficient to have the WSPR-band within the coverage of the SDR-receiver.
2. Could be that the software does not provide a "virtual local oscillator". In this case, the SDR-center frequency would need to coincide with the "dial frequency" for the respective band.

In practice, both solution would be equally easy to realize on 600m. A dead simple setup could be, using a 2.000MHz oscillator in a standard divide by four fashion, e.g. using Flip-Flops.


Oscillator

In case 1, a SDR center frequency of 500kHz would be just fine, hence, the transceiver design could be based on a simple 2.000MHz CMOS oscillator.
In case 2, the SDR center frequency would need to be 502.4kHz. This would chance the design in so far, that a XO would have to be pulled to 2.0096Mhz. A pull by 9.6kHz could be ambitious on 2MHz, even in a Pierce oscillator, but certainly worth a try.


Mixers

Keep it simple, I would very likely opt for 4066 switches. Other stuff imaginable....


Power Amplifier

We are digital, using carriers, linearity is not the primary goal at this stage. So, simple/cheap possibilities would be IRF510, IRF820 etc.


Thoughts

Frequencies are kinda low, so, this all could be done in pure CMOS, which would have the advantage of a) being low power and b) easily creating 8V output to drive the MOSFET.

I hope I can realize this before the expiry of the experimental license.

Keying by Counter

Here's another trick I used in one of my rigs, chirp-free keying by means of a counter.
In previous entries, it was indicated that counters could be used to reach a certain frequency, in particular in the LF and MF bands.
Here's an example, well, this example actually reflects the design of my 600m keyed exciter.
To reach 500kHz the signal of a 4MHz ceramic resonator VFO (74HCT04 in Pierce configuration, with some gates used as buffers) is divided by 8. This involves preferably a ripple-counter. My exciter design involves a 7493 which has got an "enable" circuit. And here is where keying is done. The oscillator, since it is using a different IC, is not affected by the keying and therefore does does not create any chirp.
This concept does not provide "click prevention", however, I believe clicking is more acceptable than chirp.