Shorting BNC Caps - What are they used for?

Discussion in 'QRP Corner' started by KD2KUB, Apr 28, 2019.

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  1. KE5MC

    KE5MC Premium Subscriber QRZ Page

    Only for a brief second or two. They are for terminating the end of a line when the intended item to connect is missing and that messes with the line.
  2. K8JD

    K8JD Ham Member QRZ Page

    BNC connectors with a 50 ohm load resistor tucked away inside them.
    I see them used on unterminated ports in a receiver multicoupler at comm sites where there is a multichannel trunked system using one antenna way up on the tower.
    Usually something like 1/8 Watt resistors because there is only micorwatts coming from the antenna multicoupler pre amps.
    The shorting ones I have seen on an unused output port on a signal generator that may have high level and low level calibrated outputs.
  3. KL7AJ

    KL7AJ Ham Member QRZ Page

    They are also used for calibrating network analyzers.
  4. KT1F

    KT1F Ham Member QRZ Page

    That's basically what they were used for on computer networks. The cable needed to be terminated with a matched load so that reflections were minimized. In other words, a low SWR.

    Reflections would increase errors on the network. I'm guessing but I think the receiver inputs in the network cards in computers etc were relatively high impedance so they didn't affect the transmission line behavior along the way. They were just sensing voltages.
  5. AF7XT

    AF7XT Ham Member QRZ Page

    Had trouble with images and reflections from the counters and the sig/gen that are also in the probe lines. Added the termination to the "tees" on the scope input and the disturbances are gone. This hasn't always been the case so YMMV.

    .5 watt may be generous. .25 might be more accurate. More likely is if they see a watt CW they heat and change value. I can verify that 10W for more than a few seconds destroys them . Ask me how I know.
  6. WI9MJ

    WI9MJ Ham Member QRZ Page

    The AIM4170 antenna analyzer uses open, shorted, and 50 ohm terminations to calibrate the unit. You can calibrate it at the unit or at the antenna end of the feedline.
  7. KC8VWM

    KC8VWM Ham Member QRZ Page

    Yup and they work great for your QRP rig like my FT-817 for example.

    Amphenol Part # RF-4700-51 (51 Ohms)



    The power rating thing is kind of deceptive. The rated specification is about non stop, continuous operation of the internal resistor itself without any heat sink attached. The metal housing provides a lot of heat sink dissipation, so don't be put off by the power rating specification. I had this one shown in the photo since 2003 and it does handle a full 5 watt output for a few minutes at a time without any problems whatsoever.

    The very bottom line here is these will and do work as an effective dummy load for your 5 watt QRP rig, but common sense needs to be used. Monitor how hot it's getting and don't overdo it or somehow expect these to be indestructible, they're not. Now if you want a dummy load you can transmit continuously for a period of 30 minutes, then this probably isn't the dummy load for you. During my own observations, these get the hottest temperature when transmitting 5 watts on VHF FM after transmitting continuously for two minutes or so. SSB transmissions barely gets it warm even after three minutes or so. The final takeaway of all this is that for casual testing purposes typically consisting of brief test transmissions lasting approx. 1 - 3 minutes at a time, I have yet to burn it out after using it for over 16 years !

    The chain is attached directly to one of the screws on the side of the rig so it doesn't get lost, and I guess it does have the added benefit of functioning like a dust cover to protect the rigs BNC connector when it's not in use. However, i'm really just using it as added insurance in the event the rig microphone PTT switch somehow gets jammed against something and the rig starts transmitting on it's own without any antenna attached while travelling with it in storage. I actually had the rig power on without my knowledge before while in travel, so I don't want to take any chances without any antenna attached. Lately, I have been removing the internal battery to prevent this, but the dummy load is extra insurance against blowing something up through some kind of unintended cockpit error.

    I can also use this dummy load to perform certain sanity checks when working out in the field with my QRP rig. Things like testing / measuring the transmitter output levels, or making mic audio level adjustments without requiring me to actually go through all the trouble of deploying an antenna outside. So it's simply an added convenience to have the dummy load available to transmit into, so I can quickly check something with the transmitter. I can also use the dummy load to test my coax feed lines for shorts and / or bad connectors without requiring any multi-meter. The SWR meter built into the rig can tell me if the antenna feed line is good or bad when tested with this dummy load. I can also use it to rule out an antenna problem, verses a feed line problem this way, so I know exactly what needs to be fixed. Nothing worse than being outdoors in the sticks and not knowing if it's your antenna, or the feed line that isn't working properly, so a dummy load takes a lot of the guess work out of that. So yes, a dummy load like this actually does have some real world uses for QRP operation.
    Last edited: Jun 17, 2019

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