40/80 vertical dipole antenna, lesson one

Tempus fugit. Sometimes it feels that someone is playing around with a time machine, but in the last two week we have seen a fair amount of excitement on and across the islands. The election drama continues with court cases, legal action brought against the Government who claim that the opposition party is trying to block the constitutional and electoral process.

Storms have passed through the Leewards, but well off our Southern coast, a bit of wind but not much rain at this QTH. September 19th is another Independence celebration and that is sometimes accompanied by stormy weather, the annual coincidence, and it seems to be already shaping up in the Atlantic. We hope that it will blow over quickly, pass us by as a depression, or just don't come at all. Nature is unpredictable but the weathermen are still trying to con us.

I am finally down to the last phase of antenna adjustment because I do not settle for any SWR less than 'one to one' at my operating frequency. This has taken me that long to get to this point because I did not follow the basic principle that any experimentation should always begin at lesson one, not at lesson 5 or 6. There are no exceptions or exemptions.

The antenna itself posed no problem for either 40 meters or 80 meters. The length and taps were okay. The solenoid balun at the feed point was also okay, but the feed line was the trouble. Not wishing to cut another length of coax cable I elected to use precut lengths. That is cool if I had measured them precisely, but I did not, and resorted to a rough hand span estimate. 191 feet, about 6 foot too long but close enough.

The self resonant frequency was in band but the best SWR on both 40 and 80 was only 1.5. That is workable if I am using an antenna tuner, but I am not, and need the SWR at 1.0. So I decided to start from lesson one. In the low band DX handbook ON4UN tells us to start with all known parameters, no unknowns, measure, calculate and compute everything, then you are good to go. When I was building the phased arrays, that is what we did, but that was then, this is now. The only unknown was the guesstimated coax feed line. So out came Lufkin.

The feed line required for this 40/80 dual band vertical antenna is one wavelength on 80 meters and two wavelength on 40 meters. If the antenna was on the closest mast the cable run would be 50% less. For 80 meters the feed line required is 173 feet and for 40 meters it is 185 feet. So my 190 feet feed line was way to long for both bands. So the precut cables were measured and joined to be as close to 173 as possible.

The Autek RF-1 was deployed but I had to put that back in the box because it did not show me what I thought it could. I needed to know what was the frequency length of the feed line plus solenoid balun, so I had to resort to the measurement method ON4UN recommends in the handbook, using the transmitter and dummy load. That method worked like a dream. I now have a switch system to add the additional twelve [12] feet of feed line for 40 meters. That will be update with a DPDT coax switch sometime in the future.

So we are at the final stage of tuning up. The SWR is now below 1.5 on both bands and tomorrow a freq run will be done, and if the wind is not high, the antenna droped for final adjustments. Contacts on the antenna as is are encouraging, but I notice a sort-of-BCI interference component on 80 meters, but inserting the attenuator eliminates that. We are less than two miles away from a 50KW am station on 820 KHZ, and about three miles away from a 20KW am station on 555 kHz. I think there is a BCI mod for the TS-940 somewhere.

I am keeping an eye on the Flex 3000, and I am awaiting the US reviews, and following up the discussion groups. The Flex 3000 lends itself nicely to my style of portable and mobile DX operation, and I am looking forward to that for this winter. By the looks of things I will have to make a decision in 8 weeks time. These are the days where one has to make decisions on facts and figures not on emotions. The fact is that I live in kilowatt alley and my next rig has to be able to handle that, from the base station or in the field.

Tomorrow I will compare the Flex 5000 and K3 to determine best value. So far the Flex is ahead because of its non disposability, which I rather like. One thing for sure I am not buying another rig made in Japan. It is rather sad that none of their rigs with affordable price tags can make it into the top 10, or should I say more precisely, can make the now acceptable minimum receiver spec DRNS 80dB@2kHz.

WOW!!! I just pulled up the Sherwood receiver test data table and see that the Flex 3000 is now there, and in the top 6. I will not rewrite this blog, but it seems that I can now make the space on the operating table for my next rig, the Flex 3000.

I am going to bed now and will dream about my Flex 3000.