The AM Forum

For what purpose would a link coupled transmitter tank circuit be tuned up into a 400 ohm dummy load to provide an unbalanced transmission line connection?

Off center fed half wave antenna for one. --  At a nominal 400 ohm point,  not in center nominally 75 or 50 ohms or so depending on height from ground, ground characteristics, etc.

Interesting lash up possibility.  Do you have a rig that has this as an instruction possibility. Antique or military, etc?

The transmitter is a BC-610 that I am just beginning to restore.  I ran across and antenna switch / dummy load, pt# SA-24, which is part of a
T-55/MRQ-2 transmitting set. The T-55 is a modified BC-610. The following link has a bit of information about the unit:
 http://www.navy-radio.com/manuals/94200/94200.1a_9-1.pdf

Based on the description the transmitter was set up to use 1/2 rombic antennas.  I was curios to what others thought of 400 ohm dummy load without the T-55 information.  It will be a interesting test to tune up into the 400 ohm and see how a link coupled balanced antenna tuner works out.

For those scrambling to find the info:  The T-55 is on Page 70.  Ahem.

I do not see anything about a 400 Ohm dummy load in the T-55 section.  Am I missing something?

I need to refresh my memory on Half Rhombics, but as I recall some of the Rhombic family antennas use loading resistors (but not 'dummy loads").  Is this what you are referring to ?

A link coupled, balanced tuner should be able to match many impedance loads, and given the era, I would expect it to be able to match 400 Ohms.  I don't see anything particularly interesting about that.  shrug.

B&W made lots of 'em.  Called 'terminators'.

73DG

http://www.w8ji.com/rhombic_antennas.htm
Here is discussion by Tom W8JI about Rhombics, Half Rhombics (Vee and Inverted Vee) antennas.  Note the link on the page to a handy design table.

B&W still uses them on their multi-band 'no tune' folded dipoles.

Ohmite sells several lines of non-inductive ceramic resistors.

I think they still make one.
http://www.bwantennas.com/

Friends with dealings in MARS/CAP/FEMA use them for local (NVIS) ALE work. They do well for that -no tuning and fast frequency changes with no extra tuner. The idea was more important before the advent of high speed automatic tuners but still works where the antenna installation already exists or where field portability is of interest.

Sorry I should of added a bit more information,the SA-24 antenna switching unit does not have a functional description of the unit on the t-55 page in the pdf.  I have not been able to locate a schematic of the t-55

The SA-23 contains an antenna switching relay, a 400 dry dummy load and a knife switch to select the dummy load or the antenna transfer relay. I will upload a picture of the switching unit this weekend.

Jeff