[Boatanchors] Re: GB> Collins R-388 vs 51J4

[jeremy-ca]

A 15' whip will have an impedence of roughly 35 Ohms plus ground loss at its 
1/4 wave resonance. The impedence decreases rapidly as the frequency goes 
lower than the 1/4 wave point which is 15.6 MHz. As the frequency goes up so 
does the impedence.

The 51J series has a reputation for poor sensivity which is partially due to 
the input design and the way many use it.

In many installations a 51Jx, and R-390 series for that matter,  is part of 
a group of receivers. These are fed from an active distrubition panel which 
contains an amplifier(s) that makes up for the signal spitting losses as 
well as providing a constant impedence to the receiver. Typical antennas 
varied from 15-35' whips aboard ship to long wires and various directional 
arrays to rhombics and large LP's at shore stations.

Whether the input is 300, 450 or 600+ Ohms makes very little or any 
difference to a receiver with a high impedence input so the balun ratio is 
not critical. Purists can easily construct one that can be switched in steps 
from 4:1 to 12:1.

One potential problem with many BA's is signal overload from nearby BCB 
stations when a big (long) antenna is used. A CATV balun or splitter with a 
5 MHz lower limit somewhat reduces that potential by having a built in 
negative "gain" (insertion loss) below the cut-off frequency. These CATV 
baluns could care less if the coax feed is 75 or 50 Ohms, the ratio is still 
4:1 resulting in either a 300 or 200 Ohm output in a perfect system.
My 51J3 and J4 were loaded with BC birdies on the lower ranges until I built 
a BC reject filter. Published IMD figures for these receivers are fairly 
poor and nowhere close to a R-390/390A.

Carl
KM1H



----- Original Message ----- 
From: "Al Klase" <al at ar88.net>
To: "Glen Zook" <gzook at yahoo.com>
Cc: <Boatanchors at mailman.qth.net>; <glowbugs at piobaire.mines.uidaho.edu>
Sent: Monday, October 01, 2007 12:56 PM
Subject: Re: [Boatanchors] Re: GB> Collins R-388 vs 51J4


> RE:  9:1 Baluns
>
> Yes, older receivers, designed for random-wire antennas generally have an 
> input impedance in the neighborhood of 400-600 ohms.  This a good 
> compromise between the resonate (quarter-wave x N) and anti-resonate 
> (half-wave x N) case.  The R-388 is designed to work with a 15-foot whip, 
> an even higher impedance situation. TV baluns are a crap shoot at HF, 
> often loosing effectiveness below 10 MHz, and you probably want a 1:9 
> anyway.  It's easy to make equivalent devices, and the required cores are 
> very useful around the shack.  You guys have shamed me into updating my 
> website:  http://www.skywaves.ar88.net/ANTENNA/antsys.htm
>
> Regards,
> Al
>
> Glen Zook wrote:
>> The article that was in Electric Radio a while back
>> about using television baluns on the older receivers
>> can be found at
>>
>> http://k9sth.com/Page_2.html
>>
>> The link is about 2/3rds the way down the list of
>> links and is entitled "Using television baluns to
>> improve receiver sensitivity".
>>
>> Glen, K9STH
>>
>>
>>
>>
>
> -- 
> Al Klase - N3FRQ
> Flemington, NJ
> http://www.skywaves.ar88.net/
>
>
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