Sunday, February 28, 2010

Configuring Elecraft K2 Crystal Filters

The Elecraft K2 has a number of unique features, one is the variable bandpass filter which can be programmed for different bandwidths. This leads to a very flexible design for CW or RTTY, but requires a bit of configuration work. For each mode, you can select up to four crystal filter configurations (FL1-FL4), including using the KSB2 module filter (OP1). The first filter configuration (FL1) is also used when transmitting.

Typical configurations use OP1 in the FL1 position. For CW, I opted to use 1000 Hz in FL1, then with the progression of 400 Hz, 160 Hz and OP1 for FL4. The 1000 Hz setting is about the widest setting that still has smooth sides from a single peak. Much wider, and one side starts to get a pronounced "hump". This filter is good for general tuning on an active band. 400 Hz is great for crowded contest conditions. I rarely use 160 Hz, but it is useful for digging out weak ones. I centered these on 600 Hz. The OP1 filter is good for tuning a quiet band, but it is harder to zero-beat stations.

SSB and RTTY require OP1 in the FL1 position. On SSB, I used 1.8 and 1.6 kHz settings for FL2 and 3, but I rarely use them, the asymmetric filter makes for harsh listening.

RTTY necessitated a minor modification. During the A to B modifications, I added 47 and 100 pF caps across C174 and C173, respectively for the BFO stability mod. I changed these to 56 and 120 pF. However, this did not shift the BFO frequencies at all. I added 3 pF from the X3/X4 junction to ground and this lowered the lower BFO frequency 800 Hz without affecting the upper frequency. The wider BFO frequency range is necessary to accommodate the higher tones use on RTTY.

I originally picked the "low tone" frequencies of 1275 / 1445 Hz for RTTY. However, the resulting center frequency of 1360 Hz, can't be used with the KDSP2 filters. The KDSP2 filters can only be set to multiples of 100 Hz. So, I switched to frequencies of 1415 / 1585 Hz, nicely centered around 1500 Hz.

For the rest of the RTTY filters, I used 1000 Hz, 500 Hz and 300 Hz. 1000 Hz is good for general listening, 500 Hz is pretty much single-signal, and 300 Hz clips the edges of the signal passband, but is useful for trying to dig out stations with surrounding QRM.

Align the filters using the standard procedures for the CW and SSB filters, the RTTY filters are pretty much the same as the CW filters, except the center frequency is different. Instead of Spectrogram, I use CocoaModem on the Mac. If you go to the Config window, CocoaModem has a nice spectrum display. You can set the RTTY frequencies as markers in the window. You can see the picture at the top of the article shows CocoaModem displaying the 400 Hz filter spectrum, with markers at 550 and 650 Hz.

There's two things to know about setting the K2 filters. First, there's a limit to the resolution of the DAC used to tune the BFO -- it may not be possible to get the BFO of each filter exactly on the right frequency. The net result is that the frequency of a desired signal may shift slightly when moving from filter to filter. Second, it's important to match up both sidebands so they look and sound the same. Because of the VCO design, the K2 inverts signals above 20 MHz, so the filter used for LSB is used for USB above 20 MHz and vice versa. (The K2 firmware takes care of this automatically)

When setting up the filters, it's helpful to try to flip between opposite sidebands (using the CW RV button), and check that both sidebands sound about the same. Bumping the BFO setting by one tick can often be helpful in getting them to match.

Similarly, walk through the filter settings (using the XFIL button) to monitor any shift in the received signal. Moving the BFO setting one tick can help in some cases, but because of the DAC resolution it won't be completely perfect. With a little patience, it isn't hard to get the filters as close as possible.

The filters in the K2 offer excellent performance in an inexpensive radio, it just takes a bit of care to configure them.

Sunday, February 21, 2010

WARC Trap Dipole

Although I've had good success using an untuned doublet on the WARC bands, I made up my mind that it would be nice to have an antenna that didn't require a tuner.

Initially, I built this antenna has a 30m / 17m dipole. The traps are made from 1.5" Schedule 40 PVC pipe, which is about 1.9" in diameter, wound with RG-59U coax. I built them in much the same manner as those I used later in the 80/40m dipole.

The 17m traps are 35 1/2" of coax wound as 4.8 turns. I trimmed these traps to 18.1 MHz. I actually built three traps, the first was a dummy form so I could figure out where I had to drill the holes to pull the coax down tight.

The 30/17m version uses 12 1/2 feet of wire in each leg, and the outer segments are 8 feet. You may have to trim the lengths to resonate the dipole in the band.

Adding 12m requires two more traps. These were made from 26 1/2" of coax wound in about 3 and 3/4 turns. Trimming the 12m traps right on frequency takes some doing, as small changes in length can shift the frequency of the trap considerably.

I inserted the 12m traps 9 feet 5 inches from the center, and then trimmed the length of the outer legs to bring resonance within the bands. I did not do a great job of measuring -- the 12m traps ended up in slightly different locations in each leg. It does not appear to affect the antenna much.

You should measure the antenna at some height, as the tuning will change as the antenna is raised. I did my initial trimming at 15 feet, and when raised to 42 feet, the resonance frequency rose over 200 kHz on 30m, and slightly less on 17m.

I built a current balun into the center mounting box for the dipole, using 10 type 43 1" ferrite beads, slipped over coax inside the box.

Since the WARC bands as so small, antenna SWR bandwidth isn't a consideration. At 42 feet, there's a little bit of a broadside pattern on 30, more so on 17m and 12m. Since I installed this antenna as an inverted V, the pattern is a bit more circular. The traps can easily handle the 700 or so watts on 17 and 12m.

The WARC bands are a lot of fun with a decent antenna. I've worked several DXpeditions with this dipole on 17m. Without sunspots, there hasn't been a whole lot of activity on 12m yet.

This sort of antenna is easy to build and install. There's no reason lack of antenna should keep you off the WARC bands.

Friday, February 12, 2010

Trap Dipole for 80/40m

At my old QTH I used an 80m untuned doublet: 125 feet fed with open wire using a remote balun. I put it up my at current QTH, shortening it to about 115 feet to fit my lot.

It worked OK for a low dipole, but it was a bit troublesome. Even with a Murch UT-2000A tuner, I had trouble using it with my amplifier. It also took time to retune when switching bands.

I decided what I needed was a trap dipole. I'd had some success building a trap dipole for the WARC bands using coaxial traps. After reading the analysis by W8JI, I still wanted to use coaxial traps, but I decided to use traps resonant off the operating frequency.

You can see the result above, about half the antenna is visible, with the feedpoint on the left, and one of the traps just to the right of center.

The traps are designed to be resonant about 6.6 MHz. They are made of 76 inches of RG-59A/U, wound in 6 and 1/3 turns around a four inch long piece of 3" schedule 20 PVC. The actual frequency of resonance isn't critical, so long as both traps are identical.

The traps are wired in series, as shown below, the inner wire (black) attaches to the center conductor of the coax, and the outer wire (red) attaches to the shield of the opposite end.

The shield of the inner end is connected to the center conductor of the outer end, as diagrammed by VE6YP.

The center section is made of two 33 foot pieces of wire. In retrospect, another foot might have been better. The outer sections started at 26 feet each. I cut off 10 feet 8 inches to bring the antenna into the band, resulting in 15 feet 4 inches. If you were to duplicate this antenna, I would start with 35 feet and 20 feet, respectively, and trim from there.

Fed with RG-8x to a height of 35-42 feet, the antenna resonates at 7.22 MHz with a VSWR of 1.5:1, and 3.76 MHz with 1:1. With the typical automatic antenna tuner in modern rigs, or the pi network of older amplifiers, this antenna easily covers both 40 and 80m. Being so low, this antenna doesn't have much of a pattern on 80m, and only a gentle broadside pattern on 40m.

I've been pretty happy with the dipole. It works well on 80m early in the evening when the propagation is short. Not bad for an antenna that's less than 100 feet long.

I hope to add a couple more traps to add 30 and 20m this summer. I'll let you know how that works out.