[Elecraft] 500 kHz Transceive Operation With the K3

[wayne burdick]

A few countries allow experimental amateur radio operation in the 500 
kHz band (600 meters). This is a fascinating and historic band that is 
used for beacons and ground-wave communications. For hams, there are 
very specific mode and power restrictions, and an experimental license 
may be required. In the U.S., see http://www.500kc.com/ and 
http://www.arrl.org for the latest news.

The K3 can be used for both receive and transmit on this band as 
explained below.


500 kHz Receive-Only Operation
------------------------------

The K3 can receive signals in the 500-kHz band if both the KXV3 and 
KBPF3 modules are installed.

The KXV3 (RF I/O module) is needed because it provides an RX ANT IN 
jack. This jack bypasses the high-pass filter in the normal receive 
antenna path. At 500 kHz, the high-pass filter--which protects the T/R 
PIN diodes--will attenuate signals by about 20 dB. There is no such 
attenuation at 500 kHz when using the RX ANT IN jack, and sensitivity 
is excellent. We measured an MDS of -133 dBm with preamp OFF and a DSP 
filter bandwidth of 50 Hz.

The KBPF3 (general-coverage module) is needed because the normal 
160-meter band-pass filter covers only about 1.7 to 2.1 MHz. The KBPF3 
includes a low-pass filter that passes signals down to 500 kHz with 
very little attenuation.

A low-noise receiving antenna is recommended for use at 500 kHz.

Diversity receive should be very useful at 500 kHz. To do this, you'll 
need a sub receiver (KRX3) that is equipped with its own KBPF3 module. 
You'll also need a separate receive antenna for the sub, oriented 
differently from the receive antenna used with the main receiver.


500-kHz Transmit/Receive Operation
----------------------------------

The K3 cannot be used to directly transmit on 500 kHz at high power. 
The low-pass filter cutoff frequency is well above that needed for 
attenuation of harmonics on this band, and other components in the 
transmit path are optimized for 1.8 MHz and higher.

However, the K3 can put out a clean 500 kHz signal of about 0.5 
milliwatts at the XVTR OUT jack (on the KXV3). This can be fed to an 
external amplifier (this is left as an exercise for the reader). 
Transmit power is restricted on this band, and is usually expressed as 
ERP (Effective Radiated Power). This means you can use high power to 
make up for the use of an electrically short antenna. Since 1/4 
wavelength at 500 kHz is about 468 feet, not many hams will be using a 
full-length vertical or dipole.

As with receive, transmit at 500 kHz requires the KXV3 and KBPF3 
modules. In this case, though, you'll need to use the XVTR IN and XVTR 
OUT jacks, with XVTR IN going to the receive antenna, and XVTR OUT 
going to your 500 kHz amplifier and a transmit antenna. If the same 
antenna is used for both transmit and receive, an external T/R switch 
will also be needed, controlled by the K3's KEY OUT signal. If separate 
transmit/receive antennas are used, it may be necessary to provide a 
PIN-diode switch to open or short the receive antenna during transmit. 
The XVTR IN jack is normally used with a transverter, so it doesn't 
have such protection built in.

IMPORTANT: In order to transmit at 500 kHz via the XVTR OUT jack, you 
must set CONFIG:KXV3 to TEST. This routes all signals through XVTR IN 
and XVTR OUT, which is why the receive antenna must be connected to 
XVTR IN in this case. (You can't use XVTR OUT for transmit and RX ANT 
IN for receive due to switching limitations.) Be sure to set KXV3 back 
to NOR when using normal ham bands.

If interest in the 500-kHz band is sufficient, we'll change add 160 
meters as an IF band for use with transverters. This would allow you to 
set up a 500 kHz transverter band, so it would not be necessary to set 
KXV3 to TEST.


Synthesizer Considerations
--------------------------

Before using 500 kHz, you'll need to make sure your synthesizer is 
adjusted to cover this band. Tap DISP and use VFO B to locate the 
"PLL1" display. If the voltage is less than 0.9 V when VFO A is set, 
contact customer support for suggestions on how to adjust it. If you 
plan to transmit on this band, connect an short (3') antenna to the 
XVTR OUT jack and listen to your signal with another 500 kHz receiver. 
Make sure the signal sounds clean.


73,
Wayne
N6KR


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http://www.elecraft.com