07 October 2013

LDG RT-100 - new 100W remote tuner

RT-100 is the codename for the latest automatic antenna tuner from LDG, and this one is the sweetest thing yet: small, compact remote tuner, handles 100W and goes into tune mode automatically when it detects a mismatch, so no need to even know it's there. Just install it at the base of the antenna and feed it with 12V via the coaxial cable that also brings RF and you're set.

Why do I think the LDG RT-100 is so great, you might ask ? Because all tuners should be remote. The transmitter is 50 ohm, the feedline (coax cable) is also 50 ohm, the only thing that is not matched is the antenna, so normally the matching must be done where the 50 ohm feedline meets the not-so-50ohm load. Sure, you can use a tuner between the transmitter and the feedline - like most people do, because it's much more convenient to have the tuner inside the shack, but this means you will have reflected power going back and forth trough the feedline since it's not matched with the load. Reflected power on the feedline creates standing waves, wich means 1. the feedline is now a part of the antenna (antenna radiation pattern deteriorates, noise pickup and interference increases) and 2. more transmit power is wasted on the feedline (6:1 mismatch doubles the feedline losses for example).

Anyway, back to the RT-100: it is already listed on the LDG website, Ham Radio Outlet has it on preorder for US$200, and there is also a thing called RC-100 wich is LDG's way of charging US$50 for a (rather ugly) box with a button, a capacitor and a choke coil.

Yes, it's meant to power the tuner via the coax cable but at least that you can build yourself - remember we're geeky tech-savvy hobbyists, dammit, LDG are nice enough to provide the schematic in the RT-100 manual.

 Pictures are taken from the LDG website. I also found a review of this thing, looks good.


  1. Re:Your review of LDG-RT100/RC100

    With the LDG-RC-100 Remote control, does the "TUNE" button turns on a connected transceiver to send QRP RF and DC to the remote tuner and initiate an automatic tune function?



  2. The LDG RT-100 manual (available from the LDG website) says this:

    To force a tuning cycle: "Turn off power to the RT-100 by switching off power to the Bias Tee, begin transmitting a carrier, and while still keying the radio, turn on power to the Bias Tee; an automatic tuning cycle will begin. Continue transmitting the tuning carrier until the automatic tuning cycle ends with the SWR settled below about 1.7, then unkey and continue to operate normally. If you are using an RC-100 controller, transmit a carrier, press the Tune button on the RC-100 for one second then release. Continue transmitting the tuning carrier until the tuning cycle ends."

    The RT-100 manual does include a schematic for a Bias Tee, but NOT for the entire RC-100.

    It is obvious from the above quote that the TUNE button on the RC-100 simply interrupts the DC power sent via the Bias Tee in the RC-100. So the full RC-100 schematic is probably the Bias Tee shown in the RT-100 manual, with a momentary-off switch (Tune button) inserted in 12VDC input line before the RF Choke.

    AD5X has two excellent and sophisticated articles about homebrew Bias Tee construction here:


    Again, I believe inserting a momentary-off switch in the DC input line and tuning as above will allow you to have the RC-100 "Tune" button functionality in a homebrew Bias Tee for the LDG RT-100.

    - Les, W9XC

    PS: the RC-100 does not have any control connection to the transceiver, so it will never be able to put the transceiver into transmit mode, QRP or otherwise. Again, simply see the manuals.