Bye-Bye Loop, Hello vertical (for LF & MW). . .
- Today I took the 4 foot tunable loop antenna down and am in the process of putting up a "Low Noise Vertical (I hope ;-)
I put the loop up in 2012 as an experiment in the hopes that I could null out interfering beacons and add to my beacon score. It was a big disappointment though. Yes, it did pick up beacons OK, but the nulls were very poor. I first tried connecting the fet pre-amp to the high side of the tuned loop, but for all practical purposes there were no nulls. I suspect that the "antenna effect" of the unbalanced loop, along with environment (just above a sheetmetal roof) was the main problem. Later I disconnected the pre-amp from the high side of the tuned loop and used a single pickup turn to the pre-amp. Yes, there was an impedance mismatch, but that shouldn't have affected nulls, just the gain. The "Q" was high as the varactor tuning was very sharp. . .
Well, that worked better, I could get shallow nulls (around 10dB or so) on some beacons, but not others. Not good enough to sneeze at, or even to actually use the thing as the "active whip" could hear most of the weaker beacons much better.
So today I took it down so that I could install a 30 foot "low noise vertical" (I hope) in it's place. I figured that I'd offer to GIVE the loop to somebody IF they wanted to experiment further with it. Well, that didn't work out! As soon as I lifted it and the rotator off the push-up pole it got away from me and went crashing to the ground some 15 feet down! The result was a crumpled mass of PVC pipe and tangled copper wire. The thing literally came apart at every glued & screwed joint! It landed top side down and the weight of the rotator and metal TV mast probably didn't help preserve it any.
Anyway, after that little escapade I went ahead and installed the vertical section on the top of the Push-up pole. It consists of a Hustler MO-3 mast section (aluminum) with an 80 Meter resonator and stinger mounted on it (I figure that the inductance of the coil MIGHT help a bit). That is mounted on top of about a 7 foot piece of 2 inch PVC pipe, sitting over the top of the push-up pole. There is a 20 foot section of 5 conductor 18ga rotator cable coming down at an angle from the bottom of the Hustler mast to an insulator at the roofline of the mobile home (angled to get it away from the metal push-up pole. The total length is right at 30 feet and gives me a 30 foot vertical above the roof. The matchbox containing the toroid (about 80:6 turns or so) will be mounted right at the roofline and provide the balanced output running into the radio room. The primary side of the toroid will be grounded to an 8 foot ground rod separate from all the station grounds.
Tonight I'll wind the toroids (80:6 antenna section and 1:1 turns in the shack), put them in suitable housings, and hopefully install that tomorrow. I'm to "bushed" to do any more today. At 70 I'm almost too old to be running up and down ladders and shoving those poles way up in the sky, lol!
The toroids I'm using right now. They're "unknowns" from a switching PS, seem to work well at LF, but later I'll purchase a couple of new ones with known characteristics. It won't be too hard to change them out at a later date.
-- 73 de Phil, KO6BB http://www.qsl.net/ko6bb/ (Web Page) RADIOS: Grundigs: S-350 (~2006) & G6 (~2011). Heathkit: GC-1A "Mohican". Analog Solid State (circa 1965). Icom: R-75 Receiver with 2 Cascaded 250Hz CW Filters. Icom: R-71A With 500Hz CW filter (circa 1986). Icom: IC-735 HF Transceiver (circa ~1990). Radio Shack: DX-380 digital portable (circa ~1990). Zenith: Royal-7000 Transoceanic Analog Portable (circa ~1969). ACCESSORIES: Homebrew LF-MF Pre-Amp, MFJ-993B HF Auto-Tuner. Homebrew 6 Hz Audio Filter. ANTENNAS: 88' Long Ladder-line fed dipole at 35 feet AGL. Active Mini-Whip at 36 Feet AGL for LF/MW. 4 Foot/side Tuned Rotatable Loop at 15 Feet AGL for LF Merced, Central California, 37, 18, 37N 120, 30, 6W CM97rh
- good luck ! I would like to suggest adding a notch filter..i have one on my old Drake R-4b. It works so well it's my all time favorite reciever. de Mike, N9LSD
- I believe the balance of a small loop antenna is indeed the key to get its figure-8 pattern with the nulls. That, along with the metal roof might explain why yours didn't show any, and why it got at least partial nulls when you changed to a separate coupling loop.Andy