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Author Topic: Loop Antenna and Common Modes???  (Read 1797 times)
AD0AE
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Posts: 76




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« on: December 23, 2012, 09:02:18 AM »

Hi All-

A couple of months ago I purchased a mfj-236b loop tuner and have been extremely happy with the results!  Living in a very noisy apartment, I have been able to make a decent number of US contacts on JT65 and yesterday I actually had an awesome phone contact using an indoor loop antenna.  

One thing I have noticed is that if I am transmitting and have the loop tuned, I notice that if I change the length of coax from 3 ft to 25 ft, I effectively am no longer tuned.  Let me define what I mean by tuned: my SWR meter is reading the value closest to a 1:1 as possible(this is done by adjusting the tuning and capacitance to match coax impedance) and I am seeing that there is maximal current that is flowing through the loop (there is a meter for this on the MFJ-236).  As I would expect, the current through the loop will become more significant if the SWR is closest to 1:1.
I guess I am wondering why the SWR change?

Now my guess is that with longer coax, I have more capacitance and inductance on the coax that  the loop tuner must match to and account for.  Is that correct?   Could it be common mode?  If it was common mode, is there a way I could verify that?

Thank you all and 73s,

Steve
AD0AE
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AC5UP
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Posts: 3825




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« Reply #1 on: December 23, 2012, 11:03:28 AM »

I guess I am wondering why the SWR change?

A textbook loop is a balanced antenna relative to ground... All portions of the wire are as above ground as all other portions. Coax is an unbalanced feedline as one side of the circuit is grounded.

If you have a perfect loop fed through a perfect Balun presenting a non-reactive load to the tuner the length of the coax shouldn't make too much of a difference. But we'll never know for sure because you don't have a perfect loop. That does not present 50 Ohms resistive to the coax. Which means the feedline is operating with an SWR that could make it go gnarly at some frequencies (that's a technical term). Further, in another thread you mentioned operating on 40, 20 and 15 Meters. Which means if the loop was near-perfect on one of those bands the odds are it would be imperfect on the other two. With a capital 'IM'.

When you use a tuner the only part of the antenna system that's 'matched' is the coax between the tuner and radio. The coax between the antenna and tuner comprise what's often referred to as a conjugate match and the coax is as much a part of the antenna system as the loop itself.

So, yeah, if you changed the size of the loop you'd expect a change in the tuning.   Same deal if the coax length changes...   Wink
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WB6BYU
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Posts: 13029




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« Reply #2 on: December 23, 2012, 11:54:14 AM »

I waded through the whole manual for the MFJ-236 and it doesn't have a schematic, so I'll
have to make some assumptions about the circuit.

It is quite likely that the difference is due to common mode, but ANY change in the
environment of the loop can change the tuning, especially indoors where it will couple
to the house wiring, ducting, etc.  If you move the loop slightly in connecting the
cable you very well may need to retune (or similarly if you bump the unit and the
capacitor shifts slightly.)

It appears that the 236 uses two capacitors:  one for tuning and one for matching.
Such circuits are inherently unbalanced and, while they can be tuned for a good
match with the proper sized loops, it is likely that there is enough imbalance to
have the case and everything connected to it (including the outside of the coax)
affect the tuning to some degree.

If so, a quick check is to wind several turns of the coax through a clamp-on ferrite
core and see if that affects the tuning and/or indicated current in the loop.
You might even find that running the radio from a battery requires different
tuning than from an AC-operated power supply because the latter may provide
a connection from the coax to the AC ground wire, which may be coupled back
to the loop via the wiring.

Using any sort of loop like that indoors is always going to be quirky due to the
interactions with the building.  My DF loops don't work indoors because the
loop couples into the wiring:  the direction that gives the deepest null is when
the coupling to the wiring is minimum rather than due to orientation of the loop
relative to the station being received.
« Last Edit: December 23, 2012, 12:03:46 PM by WB6BYU » Logged
WX7G
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Posts: 5920




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« Reply #3 on: December 25, 2012, 11:20:36 AM »

I have an MJF-933 loop tuner and WB6BYU has explained things perfectly.
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AD0AE
Member

Posts: 76




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« Reply #4 on: January 02, 2013, 04:52:11 AM »

Hi All-

Thanks for the comments back!  For the time being, I will experiment with creating a coax looped multiple times and see if that affects the tuning.  The other thing I am considering doing is attaching the mfj-259 I have to TX jack and trying to find the location on the tuning knob where the reactance is zero, while keeping the "coupling" capacitance at minimum (presumably open). 

I will experiment around.  I just want to minimize having my coax turn into an antenna!

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