Thank you for that, Tom. I have never, though, experienced intermod on
either 160 or the AM BCB.
Maybe it's the extra loss here. The preamp is not at the antenna, it's
inside the antenna control next to my radios. The F-6 coax is between ~50'
(NE/SW) and ~600' (SE/NW). And the NE and SE are in the reverse direction
on my *580' Beverage antennas made from plated steel electric fence wire.*
I should use a different incandescent lamp and drill a hole in the front
panel so I can see it.
73, Mike
www.w0btu.com
On Mon, Aug 31, 2015 at 1:43 PM, Tom W8JI <w8ji@w8ji.com> wrote:
>
> I use a high pass filter for rejection of the AM BCB band. Between 70 and
> 80% of the net power (or voltage) into my RX system comes from distant AM
> BCB signals.
>
> Without a small 5-pole highpass that starts to roll off at 1700 kHz, I can
> connect a miniature 12V 50 mA incandescent lamp (like the MFJ 1025 uses as
> a fuse) and it illuminates a dull red.
>
> This is with no attempt at matching power to the filament cold resistance.
> ...
> Always remember there are two problems. One is the absolute limit of
> in-band signal a receiver system can take. The other is the absolute limit
> of the sum of all the signals entering an overload sensitive point in the
> system.
>
> Less than one volt peak line voltage is not enough headroom to prevent IM
> products in a reasonably good system. Back-to-back parallel diodes are fine
> for Sky Buddy receivers and FT101's. A single diode opposing another diode
> in parallel will clamp at about 6 dBm if your receiver looks like 75 ohms.
> Almost all receivers will conservatively take 15-20 dBm, or 2-4 volts peak,
> at the antenna port in band.
>
> If you have a good system, you'll want something other than back-to-back
> diodes.
>
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