Help: RF at Tuner when running FT8 only on 40m

[Nashua Area Radio Society]

I have a stray RF problem and I am hoping someone can point me in the right direction.

I have a problem on 40m when transmitting FT8 at 50 watts.  There is stray RF that turns my monitor off (it will also turn it ON if it is in the off position).  It doesn’t happen on any other band and does not happen when transmitting SSB. I have removed everything except the power cord to the monitor to eliminate a potential source from the HDMI or USB cables.  I have also replaced the wall wart and am powering the monitor directly from an Alinco power supply. When I changed from wall wart to power supply there was no improvement.

The Samsung monitor has those soft-touch buttons (not a hard switch), so I am pretty convinced the 40m signal is turning my monitor on and off.

I have solved the issue but created a new one.  By using an AM radio, I managed to track down where the RF was coming from.  I heard a large static signal at the back of my tuner.  I am using a manual B&W tuner, see the manual here: https://n6pet.com/wp-content/uploads/2016/05/Barker-Williamson_VS300A-Manual.pdf.

I run a 450-ohm ladder line to the back of the tuner.  When you use a balanced feedline with these tuners (see section 1.4 of manual), you need to run a jumper wire from one of the feedline posts to a third post which activates the built-in 4:1 balun.

I put a mix 31 snap-on ferrite on the jumper wire and the problem went away.  I believe I found my RF source.

The problem I created is that the ferrite gets hot (can’t even touch) within 5 minutes of operating.

Does anyone have any clue what might be going on, or how to resolve this?

I certainly can run with smoking hot ferrites, can I?

Thank you!

Tom KC1ELF

[Fred Kemmerer]

Hi Tom,

You might consider using an external balun to match your ladder line feeder to a coax cable. Place this unit outside just before bringing the feedline into your house. Then use a short section of high-quality coax like LMR-400 to connect that balun outside to your tuner. Balun Designs would be a good source for an external tuner balun of this type.

[Nashua Area Radio Society]

Thank you Fred.  I kind of thought that might be the solution.  I took a look at the link you sent.  The description says it’s a 4:1 tuner balun for 200 to 50 ohms.  My ladderline is 450 ohms, so is this still the correct balun?  Since the balun in my tuner is 4:1, I’m assuming it is, but if I’m going to be buying something, I might as well make sure I get the best match possible. Please let me know.  Thank you!

[Fred Kemmerer]

Hi Tom,

Why don’t you give Balun Designs a call or send them an email and explain what you need. If they don’t have an on the shelf product, they will probably make a custom one for you. Lets us know how this works out when you can via a post here.

 

[Nashua Area Radio Society]

Hi Tom,

A good balun will certainly work as Fred suggested.  But to speak to the excess heat being generated… snap-On ferrites are much less efficient than the Torroid style ferrite chokes.

I would replace that Snap-On #31 ferrite with a #31 Torroid Ferrite, and put about nine turns of Rg8X through the ferrite core.  for a great source of torroid cores try this link to Palomar engineers:  https://palomar-engineers.com/antenna-products/1-1-balun-kits

Snap ons will work, but in order to stop excess heat, you need quite a long “train” of them.

 

[Nashua Area Radio Society]

Hello Tom,

I am not sure if you have solved your RF monitor issue or the balun heating issue yet. For the monitor, I would recommend supplementing additional ferrite chokes on the HDMI or USB cable.

On the choke heating. If I am correct in understanding your description and after reviewing the document provided, the choke installed on the jumper wire will heat up due to seeing full common-mode RF current on this single wire. With the switch in position 6, your RF current is traveling on the jumper wire to the interior balun connected to your feed line. Without opposing “differential mode” current the choke is attempting to impede your full RF current and quickly becomes saturated generating heat. I would suggest reviewing alternative mitigation options for the original problem.

Regards,
KC1MDH

Scott

[Nashua Area Radio Society]

Tom,

You don’t mention what antenna the feedline connects.  Just because you have 450 Ohm ladder line doesn’t mean the impedance at the bottom end is 450 Ohms, much less without reactance.  It’s possible it is much less.  It is a common misconception one should always use a 4:1 or 9:1 BALUN with ladder line.  The appropriate BALUN value is much more complicated than that.  A 1:1 BALUN may be a better choice.

The advice to use a toroid common mode choke is on the money.  However, the best place to use it is first at the transition from ladder line to coax.  Jim, W8JI, an well-known and respected authority on antennas and transmission lines has an article on the G5RV which I am guessing may be your antenna at:  https://www.w8ji.com/g5rv_facts.htm.  An excellent, and probably the best, source of information on using ferrites is K9YC.com.  Like W8JI, Jim uses modeling and scientific measurements to make his points, rather than anecdotes and Amateur Radio lore.

73, Bill, K8TE

[Nashua Area Radio Society]

Ladder line needs to have clearance from conductors for 3 to 5 “widths” around it, or more.  Otherwise you run the risk of coupling energy to the outside conductor.  At HF frequencies, ladder line is a great choice, but getting it into the shack must be done with care.

As others have pointed out, a 1:1 balun is a good transition from ladder line to coax.  Its common-mode impedance should be about ten times the characteristic impedance of the coax.  More is better, but do not allow the windings to become self-resonant.  (See _Transmission Line Transformers_ by Sevick for a detailed treatment.)  #61 material or K-type material is appropriate at HF.  Possibly #43.

The use of the tuner means that none of the transmission lines are “flat”, there are mismatches, especially if you are using a fixed doublet antenna (i.e. a dipole but not really half-wave) over many bands.  Here, the important thing is getting from a balance line to unbalanced line (coax) to get into your tuner for an impedance transformation to 50 ohms.

The jumper wire shown in the manual between J105 and J106 is the conductor of all your transmitter current, and is no place for a ferrite bead of any kind.  Killing the patient is not the same as curing the disease.  But, I think you already figured that out.

The “balun” T101 in the schematic is a bit of hack, and is inferior to a 1:1 choke balun.  It is more properly called an autotransformer.  A major difference is that in normal operation the core of T101 is seeing all the flux from the currents due to the transmitter power.  In a 1:1 choke balun, a true Transmission Line Transformer, the core only sees the “error” current, thus little or no flux, and therefore the core’s losses contribute little loss to the system.

The advice to abandon the internal “balun” T101 (“A” in photo) is good advice.  And if, where you locate the external 1:1 choke balun (“B” in photo), you ground the coax before it enters the shack, you should greatly reduce local RF currents which may be causing your RFI/EMI issues.

Respectfully,

Spencer, W2SW

 

 

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