The AM Forum

I am planning the wiring needs in the transmitter project and am wondering if I need shielded cables (or not) on the various control lines or low-voltage feeds.

To familiarize you with my build, see the attachment pic.   The PA and Modul. will be shielded in the panel space just above the completed plate supply.  Above that will be a tube window, then the tank tuning/loading, bandswitch knob panel above that.  Then a meter panel at the top.  Low-voltage supply chassis will be mounted on interior side walls.  I will therefore need to make a lot of wire runs for the needed connections. 

For example, I have multiple leads that will be routed up to the meter panel (PA grid current, PA cathode current, PA screen current, Mod. cathode current).  I am wondering if these can all be bundled together (with suitable voltage-rated wiring) since they are bypassed at the meter terminals and at the opposite ends (PA chassis, Mod chassis, etc).   or should these be run in multi-conductor shielded cable plus bypassing both ends?

Of course, I have other such low-voltage multi-lead interconnects in the control circuitry (sequencer, overcurrent trip, T/R switch, etc.).

I am contemplating enclosing the output tank with perforated aluminum sheet to shield RF from all the wiring in the cabinet, or is this unnecessary?

Thanks for your thoughts,

Steve KK7UV

Hi Steve,

Generally there is no need to use shielded wire for control voltage connections and power supply leads. They are OK bundled together.

I would not run any cables thru any high power RF tank "hot spots" and final amplifier areas without shielding, however.  Though, I have never shielded my meters in any rigs, even the big rigs.

The final  RF tank area should be clear of wires except for the HV feed and plate choke as well as the C2 loading coax.

All low level audio should have shielding but high level modulator wiring needs no shielding.

Generally, bypass caps in the sensitive areas will take care of things.  Cathode leads need proper bypassing but can be unshielded.

If in doubt, running a twisted pair for metering and other leads helps.

I have built a lot of rigs, both big and small and rarely have a problem with instability or RFI issues. In your case, keep the RF tank section shielded as well as the lower level audio and you will be fine. Enclosing the RF final area is a good idea, though not mandatory on some layouts.  BTW, notice that many of us run our RF final tubes next to the modulator tubes in a viewing window. Even so, any  high level coupling problems are minimal.

But all in all, I'll bet you could run that rig without aluminum shielding in the RF area and it will still work FB.  Just route the wiring far away from the RF -  by snaking the bundled wires along the chassis and cabinet edges and corners.

Maybe we are still overly influenced by the old 10M / channel 2  TVI days, I dunno.   Look under an old 1950's Ranger - at all the input chokes, bypassing and shielding and you will see what I mean.  Few go thru all that trouble anymore...  ;)

T

Perhaps a topic for another thread, but has anyone heard of or experienced any trouble with one's own or neighbors' digital TVs, these many years after the Digital Transition?

Shielded cables are an easy thing to do ‘just in case’.  I use West Penn 291, twisted pair plus shield, for meters, control, and audio. It’s cheap: I use it on the job and there’s always scrap lengths.  Be happy to send some out for postage.

Not shielding RF may have no immediate stability or feedback issues, but you WILL be broadcasting every “dummy load” transmission you make.  Many years ago I had an entire cross country SSB QSO while aligning my SB401 outside of its case, into a dummy load, sitting in my basement...the main drawback was that this was before I had HF privileges....

Regarding digital TV tvi, I’ve ever seen actual tvi, but I can wreak havoc in my and my neighbor’s  Samsung TVs themselves (power cycling/error messages) with my low dipole on 30m at 100w.  I turn it down to 50w and no one complains...other bands seem ok.

Ed