ridgescan,
No disrespect intended but you are arguing with little logic. To initially proclaim that solution B works so well and then ask others for advice concerning augmenting it with specifics dependent equipment (such as a diplexer or duplexer) while not even initially describing what solution A was is, well, very odd logic indeed.
Your argument is sort of analogous to one initially saying that he just found out that washing dishes with a toothbrush works like gangbusters so could someone please provide an effective way to use the same toothbrush for both teeth and dirty dishes! You might logically expect many others to point out how ineffective a toothbrush is for dish washing. But then said, "enlightened one" responds back with "But look how well it works! It works so much better that what I was using before!" To which others would logically ask: "Well, what were you using before?" To which the response is "Um, it was made out of wood...but don't you see how well this toothbrush works!" So after some more back and forths in the conversation it is finally revealed that the "wooden object" was a toothpick. I believe you can see where this is going.
Obviously the above is silly. But you asked an antenna specific question in an antenna specific forum and are surprised that you are getting the skeptical and critical responses that you have gotten from somewhat knowledgeable individuals. To use the above, admittedly silly, analogy again, they are trying to tell you to forget about using the toothbrush for dish washing and use a good brush or scrubbing pad designed for that purpose. Of course if all you have available are a toothbrush and a toothpick then, yes, a toothbrush would be a better choice. BUT...I personally do not think it is something to proclaim as an incredible discovery which requires major consideration (except, I suppose, amongst those large brush challenged indiviuals with somewhat limited knowledge of dish washing).
I used to use a scanner antenna occasionally with a general coverage HF receiver simply because I had no other choice at the time. But I expected it to work, maybe, as a shortend whip at the higher HF band and as "who knows what" on the lower frequencies; as N_Jay said, the line is then pretty much the antenna. I knew this and certainly knew it was a horrible solution but it was all I had at the time. I, personally, didn't see the need to advertise it as a great solution - it was simply "there". If a wet noodle is all you have, well...you get the point.
As N_Jay has pointed out, band specific devices such as diplexers and duplexers (I think you are really asking for a "diplexer" in this case) are designed to work within specific bands and with specific termination impedances. It is not likely that your discone + coax HF antenna will present a good 50 or 75 ohm unbalanced impedance within the lower HF band to the diplexer, splitter, whatever. So your results will be unpredictable at best.
If you are stuck with what you have and cannot put up a better suited antenna (such as a long wire, etc.) then I would suggest a diplexer with a low pass filter on one port (for the HF) with a cutoff around 40MHz and a high pass filter on the other port (with a cutoff at say 25MHz). But, again, you may find the performance not what you would expect so would consider the discone a temporary solution until you can find a better HF specific antenna.
-Mike
-but if what you say is true about the "coax doing the work" (RG8X) then why the cleaner peformance?
