[Lowfer] Problems with receiving loop system

[Clint Turner]

Interesting that the loop isn't working well as compared to antennas 
that should, by all rights, be inferior - a few things come to mind:

- The amplifier itself isn't working as it should.  A wiring error 
(transistor/varactor wired wrong, etc.) or bad device would explain this 
- and even in this case, there could still be a difference in the amount 
of signal seen when the power is applied/removed.

- With the varactor tuning, is there any apparent peaking of the signals 
from the electronic tuning at all the frequencies where you can hear 
enough signal to check this?

- Is there some sort of lightning protection on the coax that may have 
some sort of low-frequency limit (e.g. coil between center conductor and 
ground)?

The most interesting comment that you make is that the loop "hears" more 
noise from the loop, but signals are absent, which would seem to rule 
out an issue with lightning protection:  To me this implies a wiring 
issue on the loop/varactors, but that the amplifier itself is probably 
working - at least to an extent.  The voltage on the gate of Q1 should 
be zero, and flipping the leads back and forth when checking in "diode 
test" mode between the gate and ground, one should see the 0.5-0.6 bolt 
"diode drop" -  and touching a voltmeter probe at this point should 
cause a lot of noise/signal pick-up;  The voltage on the collector of 
the "upside-down" connected Q2 ('cuz it's PNP) should be at least a volt 
away from either supply rail.

Were I poking around, I would also check the varactor side of R7 (the 
junction of all of the varactors on the K0LR drawing) and verify the 
presence of the tuning voltage as well as continuity to ground on the 
"loop side" of C2, the blocking capacitors.

Having built a similar resonant loop many years ago, it definitely /did/ 
work (I prefer un-tuned, single-turn shielded loops myself) so I'm 
guessing that when you do figure it out, you will also have a 
palm-shaped dent on your forehead.

73,

Clint

KA7OEI


On 11/1/2019 3:47 PM, Zack Widup wrote:
> I tried sending this but had an attachment that was too big, so it may not
> have gone through. Please excuse if it did.
>
> I may have mentioned the problems I have had a while back, but I wanted to
> mention it again as the lowfer season for this year is about to start or is
> underway already.
>
> I built a 20 turn rectangular loop 5 feet on a side on pvc tubing. The
> turns are spaced about an inch and a half apart. I placed taps at 3, 5, 10,
> 15 and 20 turns so I could select smaller numbers of turns.
>
> I built the K0LR high-gain preamp. I am attaching the pdf file of this but
> not sure if the group allows attachments. But you may be familiar with it.
> See the second schematic with the MVAM109 diodes. My loop is about 100 feet
> from the shack and I have 100 feet of supply and tuning voltage cable and
> RG-58 coax cable running to a box at the base of the loop which has the
> preamp and tuning circuit in it. Just to make sure everything was getting
> proper connections, I put a 50 ohm termination at the loop end of the coax
> and measured 50.5 ohms at the other end in the shack. The power wire is
> supplying voltage measured at 12 volts to the preamp box, reduced with a
> voltage regulator to 9 volts inside the preamp box the circuit. I am using
> MVAM109 diodes, and the tuning voltage at the loop end can be varied from 0
> to 9 volts, as it should be. I also included the W1TAG loop ATU circuit,
> which allows me to add more capacitances in parallel with the tuning diodes
> to cover a wider range, switched with reed relays.
>
> After all of this, I am very disappointed in the loop system performance. I
> don't know what I did wrong. I have tried both my TS850S (with the LF
> attenuator circuit removed) and a Softrock Ensemble II LF version as
> receivers. The loop system seems to do OK receiving AM broadcast signals on
> both receivers. However, I cannot receive anything below the broadcast
> band. I cannot receive a local NDB on 407 kHz (CMI) at all with the loop.
>
> I connected my MFJ vertical that is designed for 40 through 10 meters to
> the TS-850 and to the Ensemble II and they both actually receive AM signals
> with that antenna better than the loop system, as well as a CW station on
> 475 KHz, the local NDB signal on 407 kHz and several other NDB signals. It
> appears that the waterfall of the SDR receiver shows a lot more noise
> received from the loop but actual signal strengths across the AM band are
> weaker and nonexistent below that.
>
> I also want to try an E-probe antenna, but I wish I knew what thr trouble
> with the loop system is. Any suggestions would be appreciated.
>
> 73, Zack W9SZ
>
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