[Elecraft] 160m loop experiment a big failure

[Rick Bates]

Small correction:  If SINGLE BAND use ONLY, use coax.  If multiband, use
window or ladder line (coax is too lossy and can't handle the current and
voltages effectively, over time, for multiband).

[A properly cut loop will be ~50 ohm resonant.  Using 300/450/600 ohm feed
will cause a mismatch unless transformed at both ends (lossy).  Using 50/75
ohm feed is more efficient in that case.  If multiband, ladder line is more
appropriate since most of the bands will not be resonant and a tuner is
required to adapt to the specific load, coax becomes WAY too lossy.]

In EITHER case, use a current based common mode choke (DX Engineering has a
nice paper on this topic in plain simple language, they also sell parts).  

If single band use only, a common mode choke at the antenna feed is
appropriate (coax fed).  If multiband, use a choke at/near the tuner (open
wire fed).

If you're asking the internal tuner to tune ladder line connected to a
single band antenna, you're already at the limits of the tuner because of
the mismatch you've created (6/9/12:1 depending on feedline).  Again, if
single band only, use RG-6 or RG-213 (75 or 50 ohm coax).  If multiband,
it's WELL beyond the limits of the internal tuner.

It isn't always possible to make everything 'earth ground' potential (upper
story room, too far from plumbing or ground rod for examples).  In that
case, putting everything at the same potential (common bonding) in the shack
is a good choice.  Earth ground is 'nice' but often doesn't help for noise
or RFI (though it is required by codes).

If simply connecting the antenna causes RFI then you have a serious problem.
Merely connecting an unused antenna should never cause any changes.  In that
case, it's time to check HOW the antennas are switched/connected.

Not knowing what your space limitations are, you might consider making a
physically smaller, coil based antenna for 160 that is farther from the
house.  Another option is one of the smaller 'magnetic loop' antennas (with
the advantage that it is rotatable offset by cost).

It has been my experience that if RF wants to bother any electronics or
cause RFI, that it's most likely going to happen on 160.

Putting yourself in the middle of that RF field might not be the best health
choice for you either, regulations or not.

Good luck, you have a big challenge,

Rick WA6NHC

-----Original Message-----
From: n5ge

How high is the loop?  If a loop is too close to the ground it will be an
NVIS antenna and send your RF straight up into the clouds.

Use coax, not ladder line.  A properly sized loop is supposed to have near a
50 ohm match if fed from a corner.

Yes, you will definitely need a choke balun.

73,
Tom
Amateur Radio Operator N5GE
ARRL Lifetime Member
QCWA Lifetime Member


On Sat, 15 Oct 2011 12:14:31 -0400 (EDT), goldtr8 at charter.net wrote:

>I set up a 160m loop in the yard to try one out from all the great 
>reading I did regarding the antenna.
>
>For me I can get it to tune with no problems except the RF in the shack 
>is terrible.
>
>I ran 450 ohm ladder line from the antenna to my balanced line 
>connections on my MFJ 962 d tuner.
>
>As soon as I connect the ladder line the RF is bad, basically everyone 
>states that I sound like I am in a jet airplane.
>
>Now there is also a bit more to the story as the loop surrounds the 
>house as that is how the trees are plus the dipole is also very close to 
>one of the edges on the inside of the loop.
>
>So I dont know if the problem is that everything is in the middle of the 
>loop, or running ladder line into the shack is a bad thing, maybe the 
>tuner internal balun is not the correct one.
>
>Bottom line is the K3 was real unhappy with this scenario so the ladder 
>line is pulled out of the shack and coiled up on the tree next to the 
>loop outside.
>
>One thing I have researched this morning is that I need a current balun 
>and I should have it external to the shack and run coax between it and 
>the shack.  Also I should have the shield grounded like I do with the 
>dipole to direct ground.
>
>So I thought I would share my current failure while I ponder my next 
>move or give up on the loop.   Ok I wont give up, YET!
>
>Cheers
>
>~73
>Don
>KD8NNU