It sounds like a new show on commercial TV. So you think you can build a 160m antenna? No? After all the 30m DX euphoria (2 direct QSL cards from the USA already less than 8 days after the QSO) and the irritated moaning about FT5GA, it’s about time to pick up the 80/160m antenna thread. Yes, that MFJ-thingy-antenna again.
Putting up an 160m antenna would be easy if I had more space and / or more money. But let’s focus on what I do have. The garden isn’t really small and I put every square meter to use. I’m doing quite well given the real estate and setup. And more money? I wonder if a Cushcraft short loaded vertical is such a good deal for Top Band. Yes it is when you don’t have any other choice. But I do have a choice. Come to think of it: the MFJ-998 cost me more than this antenna. But combining being short on garden space and installing yet another extra (vertical) antenna is simply begging for troubles. Both RF-wise (coupling) as XYL-wise (lawn czar). So I went down the other road. That means the 80/160m antenna hangs from the tower and uses elevated radials that neither man nor pet can’t touch.
As you all know I have spent too many hours already last year and again after summer to get this Low Band show on the road. It worked and I logged many new DXCC’s on 160m last winter. And I’m psyched about my 80 contacts in ARRL DX CW working as deep as Texas into the USA. Hello Top Band, meet ON5ZO! Then I had to take it down for the construction project and after putting it back up, it didn’t work. I wonder why… To get QRV I made a quick ‘n dirty 80m vertical fed with the MFJ-998 and used it with success in WAE CW last August. But the 160m and DX contest season are coming so I need to hurry. About two weeks ago (remember?) I took the whole thing down to start over from scratch and now I’m slowly… What exactly am I slowly here? Building? Measuring? Panicking? Rotting away on 7 MHz and above?
The original concept last year was: put up random wire, put up random number of elevated radials cut to random length. Insert MFJ automatic tuner ‘et voilà’ 50 ohms on the low bands, ready to take 1kW. And when the WX is nasty and the tower is not cranked up all the way, the slightly altered impedance by a changed vertical/horizontal ratio of the L-wire will also be taken care of by the tuner. Foolproof concept. Yet no proof of concept given so far. But I’m starting to know the ways of the MFJ thing. It does not work for extreme mismatches. It does work to adjust small to reasonable mismatches. That said, the mismatch can be bigger on a dipole than on a vertical. Just my luck. Experimentally proven by yours truly.
So today I added some more radials to improve efficiency (at least I hope they do) and used the installed quarter wave L wire on 80m. In fact, this setup needs no tuner. SWR is 1:1 on 3775, and about 2:1 in the CW sub-band. Well of course: it’s my resonant 80m dipole put up as a vertical! These are
measured displayed values on the K3 in the shack, so on the other end of the coax. Fair enough. Then I added a series loading coil right at the feedpoint. The MFJ could plough it’s way through the L/C combinations on 80m but it did not really work on 160m. Mind you, it was just a stock coil that I had. I have no clue about it’s L value and I don’t even know what L value would be needed. Exit random…
Time to take a more scientific approach. I always take a scientific approach when making verticals and dipoles. Not much science needed though: calculate, cut, adjust. Use antenna analyzer to check. Works for me every time. What was I thinking? That all these weird random and extreme impedances could be matched by a finite number of L/C combinations switched by tiny relays? And then take a QRO kW and smile at me? So I lengthened the L wire to about a quarter wave on 160m. My theory got proven: this messed up the 80m operation. Because of the extreme mismatch the MFJ-998 could not find a match. But it did and worked like a charm on 160m. Even with about 900W. The nice thing is that it works QRO on 160m without the ferrite core current choke. I only use the DX-Wire coax chokes with the ferrite beads slipped over the coax.
So where do we go from here? I restored the 80m operation. So it works QRO on 80m now. I will measure the impedance then and see what coil needs to be added to load the thing on 160m. Then make a loading coil and a use switching relay to add / remove the coil for 160 / 80m operation. The coil may be off, the MFJ will adjust as long as it’s a ballpark value. I could prune first to get it better for 80m CW operation so I can bypass the tuner. It’ll take care of business when I’m in the odd SSB contest on 80m. Alternative to the coil and the relay might be a coax trap. I assume a good coil will be better, especially since it’s out of the circuit on 80m whereas the trap is active on 80m. I need to think over how exactly I will try to fix the problem. And maybe consult some experts? But at least there is a plan!
Apart from this high priority antenna work, the K3 kit has been waiting four weeks to get assembled. I need to put the kit together ASAP. And order a microHAM cable for the K3. I still need to stick some 60 labels on QSL cards and get these out to the UBA QSL bureau. That has been on the to do list for 5 weeks now. And I’d like to make a small RX antenna for 80/160. And maybe get on the bands and work me some DX? And fill in / send off my application for WRTC 2010 Team Leader.
That said, I always write this for my own pleasure. And as a kind of archive for myself. I like writing. Maybe I should write a book too one day. About my own low band antenna HI HI. I never imagined people reading this. But the stats counter tells me I get a lot of hits. And the last couple of days I got quite a lot of positive feedback from people I know telling me they like reading this. I hope it’s sincere but anyway: thanks for reading and 73! Oh yeah, I was just joking about WRTC 2010.