Sep 26, 2017

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MFJ-1798

Hmm....when you google it, you come up with entries that refer to it as "the monster". Is that really a good thing?

I found one of these 10-band vertical antennas (80, 40, 30, 20, 17, 15 , 12, 10, 6 and 2) on Craigslist for a price that I couldn't pass up (less than a third of new). So I jumped in the TJ and took a drive, returning home with a partially disassembled antenna scattered throughout the Jeep.

First of all, this antenna is WAY larger than ANY picture can do justice. We're talking more than 20' tall - and that's bigger than you think.

After getting it home, I've re-assembled it and found that it's basically complete - except for a pair of 6' long 3/16" aluminum rods which are supposed to provide loading on 20 meters (and (40/80 meters as well). Those are available locally for less than $5 total, so I'll pick them up this week.

As far as mounting goes, I'll be putting this at the peak of my roof on the south end - supported by a 13' pole standing on my southern hip-roof and tied to my southern roof peak.

Reviews say I should guy-line this antenna. I'm not sure if I can achieve that, but I'll give it a shot.

The antenna was a bit corroded (aluminum oxide - no damage), so a little steel-wool cleaned that up and provided for a better connection between components. The prior owner said it had been up for about 8 years, so it's in GREAT shape considering that age here in the rainy northwest!

I assembled it on a tripod in the backyard, with the help of a 10' ladder. A smaller person might want a 12' ladder (I'm 6'11", so I could reach the capacitance rigging on top easily with a 10' ladder).

Once assembled, I used a borrowed antenna analyzer to check resonant frequencies - recording the frequencies that presented a 50-ohm load with a SWR of less than 3:1.
The initial numbers came out with 8 bands below 30MHz as follows:

  • 4.1500 - 1.7:1
  • 8.312 - 1.2:1
  • 17.962 - 1.6:1
  • 23.435 - 1.6:1
  • 24.205 - 2.5:1
  • 25.082 - 1.35:1
  • 26.600 - 2.75:1
  • 29.040 - 1.7:1

Well, it's great to have 8 bands, but when NONE of them are even close to a Ham band, that's a bit less exciting!

So, I looked at the documentation and discovered TWO things. First, I had installed some of the capacitance rods incorrectly on the base loading coil. I also discovered the previously mentioned 20-meter loading stubs.

Swapping the loading rods around properly on the loading coil helped a bit - but I didn't have access to two 6' 3/16" rods to replace the missing loading stubs for 20-meters. I did, however, have a aluminum yard-stick, which I strapped at the point where the 20-meter loading stubs were to be installed and it made a difference - dropping the resonant frequencies closer to where they belong.

Without the yard-stick, but WITH the capacitance rods properly oriented, the resonant frequencies measured:

  • 3.4520 - 1.6:1
  • 6.8285 - 1.5:1
  • 9.9140 - 1.2:1
  • 18.300 - 1.5:1
  • 21.265 - 1.5:1
  • 24.197 - 2.25:1
  • 24.793 - 1.2:1
  • 26.535 - 2.9:1
  • 28.980 - 1.7:1

So, all in all, that's looking a bit better. I could at least (in some cases) tell what bands were *supposed* to be resonant. I think adding the 20-meter stubs will bring them all into the realm of "close".

I've got a foot mount for the mast on order - as soon as it arrives, I'll be able to get this puppy up. Between now and then, I'll be tuning things - hopefully with 20-meter stubs as of tomorrow!

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