Jul - The R-390A Frequently Asked Questions Page

switch positions to ground. Don't know if that is necessary, but the goal is to have all receivers fall
absolutely silent when they are not connected to an antenna. One reason is to insure that the power line
and local devices such as computers don't find themselves acting as signal sources.
The only way I have discovered is use of balanced feeds into the receiver. I run unbalanced (coax) line
to the set, then ground the coax shield to the receiver chassis, and connect a balun primary to the shield
and center of the coax. The balun secondary is connected to the balanced inputs of the receiver. Short
leads are very important.
I use TV baluns (300-75 ohm) on some receivers where the AM band isn't very important. From
measurements here, they don't work very well below 2.5 Mhz or so, and really dive below 1 Mhz. That
can be an advantage, and usually the receiver gain can overcome performance on 1.8 Mhz, where 160
meter operation appears normal. A homespun balun is used on the R-390 that works over a wider range
of frequencies, in fact I am still suffering overload on the AM broadcast band. The antenna is a multiband,
parallel dipole where 160, 80, 40, and 20 meter dipoles are all connected to a single RG-213
feedline.
The grounding system here should be ok, the radio room is only three feet or so above ground. I run 2-
1/2" copper strap to two ground rods and to the house water pipe where it comes in from the street. The
house is old, so there should be metal at least to the meter.
Receivers are dead on all bands except the AM broadcast band, where some stations can still make it
weakly into a receiver (use Hallicrafters SX-62, HRO, BC-779, AR-88, R-390, TS-440)
The question is can you silence your receivers by switching away the antenna, and if so, what approach
to you take? 73 de Bill, AB6MT
From terryo@wort-fm.terracom.net Sun Jul 7 19:19:59 2002
Subject: [R-390] Twinax feedline available
I have three pieces of twinax feedline with connectors. If anyone is interested I can measure them out
and get back to you with a price. Best, Terry O'
From dallas@bayou.com Sun Jul 7 20:28:48 2002
Subject: [R-390] R-390A 10 dB S+N/N AM Sensitivity
There has been a lot of confusion about how to measure the AM sensitivity of an R-390A.
Unfortunately the manuals have contributed to this confusion. The 1970 Navships 0967-063-2010
manual has a sensitivity measuring procedure on pages 4-2 and 4-3 which involves setting the
signal generator (URM-25D) to minimum output. This is equivalent to the method of turning the
signal generator on and off which is used at several web sites to find the 10 dB S+N/N ratio.
However, the Navships manual does not mention a 10 dB S+N/N ratio, but rather a 10 dB rise,
which it is. What the Navships and web sites measure is the 10 dB S+N1/N2 where N1 is the nosignal
receiver noise, and N2 is the noise due to the signal and receiver. Also, the 50 ohm
impedance of the signal generator is not matched to the 125 ohm nominal (100 - 300 ohms)
antenna input impedance (through a UG-636A/U and UG-971/U) of the R-390A. Consequently,
the signal generator reading is not the number of microvolts that appears across the R-390A
antenna input. The Army manual TM 11-5820-358-35 gives a Sensitivity Test, not a procedure for
measuring the 10 dB S+N/N ratio. The earlier Army manual TM 11-856A in paragraph 166 has
what it calls an AM Sensitivity measurement procedure. However, there are at least two things
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