most fan dipoles (e.g. DX-CC from alpha delta, if you want a commercial unit)
use the same element for 40 and 80 with a sort of trap.
And if you did have a 40/80 fan dipole, the two elements would be so close
together that they might as well be connected at the feedpoint.
If you're putting 1 kW into one of the dipoles, you want around 60 dB isolation
(i.e. 1 mW into the receiver on the other one, and trust that the receiver can
take 0 dBm into the front end).
Just as a note, there's two sort of approaches on this - one is basically a
pair of bandpass filters, one for 40, one for 80. The other approach is a low
pass/high pass combination. There's advantages and disadvantages for each.
On Tue, 18 Nov 2025 11:47:21 -0500, <john@kk9a.com> wrote:
W8FN’s QRZ pages listed his 40m/80m antenna as a fan dipole, not a trap dipole.
I am sure most people on this towertalk list can figure out how to separate a
fan dipole into two antennas with two feedlines. If a diplexer is a better
option than go for it, I was just suggesting another option.
John KK9A
From: Jack Brindle
Sent: Tuesday, November 18, 2025 9:42 AM
To: john@kk9a.com
Cc: towertalk@contesting.com
Subject: Re: [TowerTalk] Looking for a HP 80/40 Duplexer
Two feed lines on one antenna is definitely not a solution. The only isolation
would be whatever loss is in the feed lines. Basically you would be putting
high power directly into the *other* receiver. I doubt that is survivable.
Pavel has a page that specifically discusses his 80/40 diplexer. It is
definitely worth a look. That is at:
https://va6am.com/2017/01/01/4080-band-diplexer/
73,
Jack, W6FB
On Nov 17, 2025, at 10:04 PM, john@kk9a.com wrote:
As others have stated I am sure that VA6AM can make a 80/40 duplexer for you
however the easiest approach would be to just run another feedline to your
fan dipole.
John KK9A
Randy Farmer w8fn wrote:
I'm investigating upgrading my station from its present 500W capability
to 1500W. I've located sources for most of the up-rated bandpass filters
and other hardware required for the project, but can't seem to find a
source for a duplexer to split a 80-40 dipole for SO2R use. I'm
presently using one of the Low Band Systems 500W 80/40/20 triplexers for
this function. LBS apparently makes a 1500W 80/40 duplexer, but it
appears the LBS stuff is no longer available in the USA. Does anyone
have or know of a suitable 1500W unit? I do NOT need it to be rated for
100% duty cycle since I'm only interested in CW and occasional SSB use,
but it needs to hold up under contest conditions with a full power
amplifier.
4O3A makes a nice 20/15/10 triplexer, but they don't have any high power
80/40 units. Antennas-Amplifiers (distributed by DX Engineering) makes
nice HP bandpass filters, but not a triplexer or duplexer. VA6AM (also
distributed by DX Engineering) doesn't have any high power devices.
73...
Randy, W8FN
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