As others have pointed out, with the two antennas in such close proximity,
power going into one antenna would go directly into the other. Without
isolation, the second receiver front end would blow up. You really need to have
pretty low signal into the receiver to prevent problems. And, assuming you want
to do SO2R, listening on the second receiver while transmitting on the first,
you need to have no more than about -110 dbM of signal into that receiver. It
doesn’t really matter that the incoming signal is in a separate band, much more
than that will cause defensing of the rx.
The right way to do this is to use a 40/80 diplexer with 80m and 40m BPFs
(which I believe Randy has). That puts a lot of loss between the two antennas.
Now, for reference, many of us are using this same approach on the high bands.
My station uses a C3S-JK for 10-20m, a VA6AM triplexer and VA6AM HP filters for
80 - 10m ( and a 160m LPF if I ever get a 160 antenna in place). I can transmit
with lots of power on any band without having any desense on other bands. That
makes running on 20m and S&P on 10m quite doable, even though they are
harmonically related.
One other thing. In my discussions with Pavel, he recommended not using a trap
antenna with the triplexer. The reason is that the traps will actually produce
interred images that will affect the receive band badly. Thus I took down my A3
and replaced it with the C3S-JK. Not only does it play well with the triplexer,
but it has shown to be a much better performer than the A3.
W2VJN has the authoritative document on the topic, named “Managing Interstation
Interference”, although the devices mentioned are pretty much out of date. This
book is now available for free on the internet.K9YC wrote a paper for NCJ that
corrects some of George’s information. Look for this at K9YC’s web site. AC0C
has an interesting web site that discusses the topic, with the math that shows
the needed isolation (https://ac0c.com <https://ac0c.com/>). And, to boot,
VA6AM has quite a few excellent technical docs discussing all of this. Just go
to his web site (va6am.com) and start looking around. You can spend a lot of
time at these sites and will learn quite a bit. This is time well spent,
especially since applying the knowledge will save your transceivers from repair
trips.
73,
Jack, W6FB
> On Nov 18, 2025, at 10:47 AM, 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 <jackbrindle@me.com <mailto:jackbrindle@me.com>>
> Sent: Tuesday, November 18, 2025 9:42 AM
> To: john@kk9a.com <mailto:john@kk9a.com>
> Cc: towertalk@contesting.com <mailto: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 <mailto:john@kk9a.com>
> <mailto: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
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com <mailto:TowerTalk@contesting.com>
> <mailto:TowerTalk@contesting.com>
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com <mailto:TowerTalk@contesting.com>
> http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|