Monday, February 28, 2011

The T2SFM Antenna

The T2FD (aka TTFD, Terminated Tilted Folded Dipole) may be know to many of us. No, I am not going to discuss the T2FD here. It is just mentioned since it forms the basis for the following design considerations.
The only thing we should keep in mind, the span of a T2FD is 30% of the lowest wavelength, not 50% as a regular folded dipole would be. The distance of the fold is 1% of the wavelength.

In a country as densely populated as Holland, one needs to think of room saving antenna solutions regularly. The goal of the game is hence to reduce the span of antenna setups.

The aerial I am thinking of needs to the at least the popular QRSS bands 80m - 30m, possibly 160m and 20m.

First design step, the a quarter-wave radiator for 30m and tilt it. The length of such radiator would be 7.6m.

Second step, a trap terminating the 30m radiator. This trap also acts as loading inductor for the lower frequencies. As a fist assumption, I was randomly picking 10µH, the resulting capacitance is 25pF.

Third, the folding distance is chosen to be 50cm, reflecting an operation wavelength of 50m, which is somewhere between 40m and 80m obviously.

The last and remaining bit would be the second "T" of the name, i.e. the termination. I figured 600Ohms could be a good value to start from. When putting this into reality, one needs to remember that the termination resistor should not be induction and should be rated at least 30% of the power applied to the aerial.

There the MMANA script a hacked in:

80m - 20m T2SFM
0.0,    0.0,    0.0,    -7.0,   0.0,    3.0,    8.000e-04,    -1
-7.0,   0.0,    3.0,    -7.0,   0.0,    3.5,    8.000e-04,    -1
-7.0,   0.0,    3.5,    0.0,    0.0,    0.5,    8.000e-04,    -1
0.0,    0.0,    0.0,    0.0,    0.0,    0.5,    8.000e-04,    -1
1,    1
w1b,    0.0,    1.0
2,    1
w2c,    0,    2.17,    114.0,    0.0
w4c,    1,    600.0,    0.0
800,    80,    2.0,    1
2,    0.0,    1,    600.0,    120,    60,    0.0
Mod by Joachim, PA1GSJ 2/28/2011 9:55:09 PM
Created by Joachim, PA1GSJ 2/28/2011 9:08:01 PM

The first simulations look promising, still a lot room for optimizations. Would be time to also start putting stuff together and try out if this contraption is any good.

Had another thought about the 30m trap. The easiest way to make a trap would be a coax trap. With a diameter of 4cm, a length of 4cm a coax trap would have a self-inductance of 2.17µH and a capacitance of 114pF. Purely mechanical, 117cm of RG58 are required in order to form 8 turns on a 4cm diameter.

Thursday, February 24, 2011

Inexpensive Small Computer

Remember when I was writing about a "grabber viewer gadget" some weeks ago?
Well, the thing can be used for more, apparently, since it most likely was not build for grabber-viewing anyway ;-)
First, lets have a look what a minimal setup can look like...

The cost of the total setup is about €150.- for the ARCHOS 7 HT (8GB SSD, microSDHC, WiFi), €20,- for the wireless mini keyboard and €5.- for the USB gender-changer. The USB cable and the power supply are provided with the ARCHOS.

The only thing one has to do is to enable the USB HOST MODE in the ARCHOS' configuration menu. As soon as a pointing device is recognized, a mouse pointer is shown on the screen.

Amongst USB accessories I tested were a Micro Hub, a card-reader, thumb drives, a keyboard with built-in Hub, mice.

There seem to be ways to toggle to an alternative window manager. Android is perfect for touch-screens, operation, however when used with keyboard and mouse, the advantages of Android don't really help.

Monday, February 21, 2011

Propagation JO29 --> JO22

Most interesting! Two Norwegian stations, LA5GOA and LA9BEA (neighbors), running MEPTs from JO29.

The locations of the two stations, as googled, are:
Steen Erik wrote on the KnightsQRSS list "LA5GOA and LA9BEA live about 15km apart on the island Karmøy on the west coast of Norway". Seen from my QTH, both stations share the same heading and hence, LA9BEA is by 15km closer to my location than LA5GOA.

And here comes the interesting part, not surprising however. Both transmitters create comparable field strengths in my humble grabber (located in JO22DA).
The following spectra were received by my 30m grabber setup: 30m-DCTL & subharmonic direct-conversion receiver. Note, the receiver does not employ AGC circuitry.
Please observe the different field strengths caused by the two friends and enjoy our ionosphere in action!

Thursday, February 17, 2011

Just an Idea

From VHF and UHF operations we know to use preamps at the antenna feedpoint as to improve SNR. SHF and EHF amateur radio operation is usually done by having the transverter at the antenna's feedpoint, since losses in the transmission line would eat up the signals completely...

So, what's the idea? Well, in QRSS, we also hunt for the best possible SNR. So, why not moving half or more of a QRSS station's circuitry to the antenna?
Half? Yes, I would prefer/advice to have frequency determining blocks, i.e. oscillators, in a controlled environment, e.g. the shack.

The basic trick on all of the following would be feeding DC from the shack to the device at the antenna, whatever the device will be. Feeding is done in the well known way for active aerial or mast-head preamps.

Easy things first, lets have a look at transmitters. The only bit that needs to be at the antenna feedpoint would be the final amplifier. The only thing to watch out for would be the Ohmic loss of the transmission line, since the final stage will require some current. If one cares about the voltage which the final is driven from, one should take this transmission line DC resistance into consideration.

Reception is somewhat more challenging. Why do it in the first place? A preamp could do that. Yes, it could. However, some designs (e.g. w/ the NE612) do not really need pre-amplification...
Here's a look at the different cases:

Direct Conversion
In this case, the LO would be in the shack, and the whole rest of the receiver remote at the antenna location. It should not pose any problem to combine/split LO-RF, RX-AF and DC. However, the supply DC can carry mains hum residuals. A decent high-pass will be necessary before the AF can be fed into any sound-card.

Supersonic-Heterodyne (Superhet)
In such a scenario, front-end, mixer and IF filter (xtal-ladder) would be distal. LO, BFO, product detector and AF-amp would proximal. It is conceivable/advisable to add a second xtal-ladder to split IF and BFO frequencies. The downside would be the more complex frequency splitting at the distal part. A crystal filter would come handy here, and therefore, a "cheap crystal combination" would be recommendable for such a design, however tempting a DDS design would be.

Now to the tricky stuff... RX/TX-toggle could possibly done by polarity reversal, e.g. by diodes and relays.
I would like to leave it to your imagination what would be possible...

This may not be necessary to think about at all...
However, in case of reception, having all the low-noise stuff at the distal location, one may consider using cheap twin lead cable to connect the remote head with the shack.
For transmission, the obvious advantage would be that the final could be matched to TX-aerial, whatever it may be in balance and/or impedance.

Thursday, February 10, 2011

JUMA-RX1 kit arrived

Unboxing of a kit... I would usually not make any noises about. However, this time it seems appropriate to share this experience with you.

SRAT sent the kit in a solid cardboard box with more than sufficient padding. I received an email the day the kit was shipped, with tracking info! The shipment was tagged "1st class - PRIORITY" and "RECOMMANDÉ". I am impressed!

The 1st class treatment continues inside the box, carrying a bag containing a TEKO casing. The kit itself is contained inside the TEKO casing:

JUMA-RX1 receiver kit
Note, the kit supplies you with everything you could possibly require... even rubber pads.

Wednesday, February 9, 2011

JUMA-RX1 possible mod

To the time of writing this post., my JUMA-RX1 kit is still waiting with postal services. Will be pick the kit up tomorrow....

Time to look into some possible modifications. Why, you may ask, why the hack modify a well thought through design? Vy vy simple, the receiver, due to the simplicity of the design, is limited to bands 40m and lower, i.e. 100kHz to 7.5MHz.

So, what is that constraint all about? The design uses a 16F819 as controller. This PIC is limited to a 20MHz clock frequency. The clever bit of OH2NLT's design is to use the 16F819's clock for the AD9833 DDS too. This however limits the DDS output frequency. One possibility would be to have the DDS running at another clock frequency. Such a modification would however require reprogramming of the firmware, in order to adapt to the new DDS clock. Possible, ok, but somewhat intensive.

OH7SV designed a direct conversion receiver with an active switching mixer. Absolutely nothing wrong with this design. If we sacrifice the 136kHz band, we could gain the 30m and 20m bands by doubling the LO. The frequency coverage will move from 100kHz-7.5MHz to 200kHz-15MHz.
There is one downside to this game, the LO frequency steps are also doubled. The 10Hz minimal steps would result in 20Hz QRG change... also frequency readout will have to be doubled to determine the QRG.

Here would be my preferred solution: introduce a second front-end having a low-pass filter, a subharmonic mixer and a low noise op-amp AF preamp. A dual switch toggles the antenna input and the preamp output between the original and the additional circuits. Extracting signal to the subharmonic mixer may require an additional adjustable driver.
Such a mod will preserve 136kHz operation and better fine tuning in direct conversion mode and offer improved performance and wider coverage in subharmonic mode.

Should there be no need for 136kHz and/or 10Hz steps, the whole front-end could be modified easily by changing low-pass filter components and adding a frequency doubler between the DDS and the RX boards.
In case subharmonic advantages are on the wish-list, the MUX-mixer could be replaced by a RA3AAE-mixer (don't forget the low-pass!). In this case the differential preamp should be modified too.

I am sure that even more possibilities would be obvious to the skilled artisan.

Stay tune for practical experience, as soon as I picked up and built the kit.

Monday, February 7, 2011

Grabber Viewer Gadget

This is actually a little bit off topic. For various reasons, I decided to buy a tablet gadget. Several options were thought through:

iPad: too big, too expensive, no USB, no memory card
Galaxy Tab: GPS, too expensive
Archos 5: 160GB HDD, 3G, GPS, screen too small
Archos 70: 250GB HDD, no memory card, sold out :-(
Archos 7: cheap, just 8GB storage, nevertheless: bought

Yes, initially, I opted for the Archos 70, in particular for its 250GB HDD. However, the Archos 70 was sold out... Moreover, the Archos 70 carries ballast I don't need, such as a webcam and BlueTooth.

So, there we go, the Archos 7 home tablet it is (for the time being). Why am I posting this on my RF blog anyway? Very simply said. This gadget allows me to observe grabbers conveniently when being in reach of an accessible WLAN. The Android 2.1 GUI allows for putting URLs on a virtual desktop for easy access. Hold the device vertically, i2NDT's compendium fits perfectly on the 7in screen, hold it horizontally, an individual grabber spectrum will fill the screen for comfortable observation.

The Archos 7 home tablet is big enough as to not being fiddly to operate (virtual keyboard size), yet small enough to carry about.

Thursday, February 3, 2011

You don't always get what you want, but you do get what you need!

That at least was the motto of my ex. Very respectfully I would like to point out that we are not together anymore for a reason, we are still friends however!

The same applies to the PFR-3 paddles... initially I wanted those, since they were fitting the PFR-3. A week after I ordered, I learned about Jerry's (W5JH) paddles, which equally attach to the PFR-3. However, having ordered the original paddles, I postponed any additional order.
Until I learned (two months after ordering) that I wont get any PFR-3 paddles...

Jerry's kit comes with two beautiful black anodized aluminum plates and three brass bars. Those five parts are machined and finished to the highest quality standard! Believe me, the manufacture of those parts is really really good!
The kit practically built itself!

So, I didn't get what I initially wanted... however, regarding the rock solid quality of Jerry's paddles, I got what I needed!