A club I belong to recently auctioned off some items to members that came from the estate of a former member, and I acquired a Kenwood TS-940s transceiver for an extremely low price. This is a 250W system that operates on 160 meters up to 10 meters. It was a top end system for its time, introduced in 1985, selling at a price tag, in todays dollars, of about $5,000.
It had all the optional filters installed (both CW and AM) that improve reception of weak and noisy signals by tailoring the width of the signal to block out adjacent noise. I now have to build an antenna and put this into operation, something I didn't plan on doing. My defense is that I was seduced by the extreme bargain.
I decided to improve its operation, taking advantage of more modern computer control and digital signal processing capabilities. I am connecting it through a pan adapter to a PC for that purpose. A pan adapter takes the intermediate frequency output of the transceiver (around 8MHz) and processes about 200KHz on either side of that frequency.
The pan adapter (LP PAN 2) provides two audio frequencies representing the I and Q, in phase and 90 degrees out of phase signals, into a Xonar U7 USB based sound card which digitizes those signals. Using the I and Q, software can act as a spectrum analyzer, showing the 200KHz spectrum surrounding the current tuned frequency of the receiver.
I can immediately see on a screen where signals exist and their relative strength. It also shows the nature of the signal (e.g. single side band or CW or slow scan TV or radio teletype). This spectrum can be shown over time as a waterfall display.
The software will do much more sophisticated noise filtering and signal processing than the state of the art back when the Kenwood was built, which is applied and the PC speakers used to output the cleaned up and stronger audio.
As well, the software controls the Kenwood radio, switching bands, modes, tuning and anything else I want it to do. Thus, if I see an interesting signal on the waterfall display, I just click on it to tune the radio.
To control the radio remotely, Kenwood provided an optional AUX board, the IF-10b, which is rare. Its outputs are TTL serial but would need voltage conversion to connect to a real serial port. Since I didn't expect this was in my radio, I ordered a third party PIEXX board that provides true RS232 and connector.
As I opened up the system and removed parts to get to the mounting point for my PIEXX board, I discovered that this radio already had the IF-10b. I could have saved money with an inexpensive RS232 level shifter board, but perhaps with the cost of DIN connectors to use the existing AUX ports on the radio, the cost difference isn't that high. Plus, I can sell the original IF-10b board on eBay.
I also noticed that this radio has the auto tuner installed. The model of the radio, TS-940s, means it had no autotuner. The model would be TS-940sat for that capability. It appears the prior owner retrofitted the autotuner, however, so that is another unexpected plus.
An autotuner will adjust the electrical characteristics of the connected antenna to reduce wasted energy and reflected signals coming back from the antenna. This is termed SWR, but to simplify things this autotuner adjusts the impedance of the radio to match what is sees from the antenna.
It does this by adjusting a variable capacitor and a variable inductor, using two servo motors, seeking the closest match possible. This autotuner can't match every possible antenna impedance, but works for a wide range of reasonable antennas. A push of a button and the motors whir as it tunes up to the lowest SWR.
My home location is very challenging for erecting HF antennas. Normally one would either erect a large metal tower with rotating beam antennas up high, or string long horizontal dipole antennas. At 160 meter band, the dipole would need to be 80 meters long, quite a bit longer than my property. It would be very very hard to squeeze a dipole for 80m on a 123 by 73 foot lot.
Horizontal antennas, whether a rotating beam up on a tower or a dipole, are directional. They deliver more power relatively in one direction than the other, whereas vertical antennas are omnidirectional.
Output power of 250 W is radiated evenly in all directions with a vertical, whereas a horizontal will appear to be about 4X as powerful in its best direction compared to the vertical. My lot is aligned so that its long direction would cause a dipole to be aimed north and south, instead of west/east where more of the potential contacts exist.
The lot is small enough and the rear is cement with a swimming pool, so no room for a large tower. Further, there are 12KV electric lines running above my back fence, quite lethal if any antenna were to fall onto them. This severely restricts possible placement of anything vertical from the ground, whether a tower or vertical antenna.
Even my front yard, which would look terrible with a metal tower, has very tall trees in front whose canopies cover much of the front. Trees absorb RF and interfere with antenna characteristics as well.
That leaves a vertical antenna as the option. The problem with a vertical antenna is that it requires a good RF ground plane underneath it to operate properly.
The usual ground plane for a vertical antenna is perhaps 90 radial wires laid along or just beneath the ground at 1/4 wavelength - so 40 meter diameter circle for the 160m band and 20 meter for the 80 meter band. This is impractical on my lot, even if I didn't have the concrete and pool in the way.
The alternatives to the full ground plane produce a less effective antenna. It can reflect quite a bit of RF power back down the shield of the antenna feed coax cable, which can cause burns and lots of interference in the house.
There are tricks, each with its corresponding compromise downside, such as using tuned rods that electrically appear to be 1/4 wavelength but are actually shorter. One can have a number of elevated rods partway up the vertical antenna mast, but at the bottom of the active section, called counterpoises. These still stick out quite a bit horizontally, but would be up in the air on the mast.
I would need the vertical antenna with counterpoises to be above my house due to the restrictions of the lot and the power lines. I am still studying the best places to put those. In the interim, I will make due with a temporary vertical antenna called a random wire type.
This is a 33 foot wire which I will hook to the top of a telescoping fiberglass pole, held on the side of my building at a location where it can't fall onto the power lines. At the bottom is an impedance matching box and to which I will hook some wire lengths as counterpoises.
I bought an external antenna tuner which handles more severe mismatches, to tune this random wire to my transceiver for this interim operation period. There are various techniques including boxes called artificial grounds that will minimize or eliminate any RF coming inside on the feed line.
It will be inefficient and perhaps require lots of tuning as I switch bands, but should let me operate on all my bands while I work on a more permanent HF antenna using a vertical with counterpoises atop my house.
This illustrates how quickly a bargain can spiral into significant spending and work - antennas, tuners, pan adapters and all the rest of the gear that goes with the very inexpensive transceiver.
No comments:
Post a Comment