5
« Last post by NQC on Today at 15:56 »
Hey All,
More random comments :
Quite an interesting concept to think of a loop as a "pre- selector". Also ,a loop certainly may not be a "substitute" for a serious outdoor wire antenna (of any type). A supposed rule of thumb ( not sure if exactly true or not ), If- a loop has ie, 100 ft of wire, even if well built,it supposedly grabs only 1/10 of the energy of a well built outdoor 100 ft long antenna (comments ?). But Q and directional ability are the advantages.
I have used both and REALLY liked having both at my disposal,way back when. Unfortunately, any serious outside wire antenna is now out of the question for me .So it's loops or pretty much nothing.
I suppose each QTH is different re the type of noise encountered, operator's "goals" , etc. One size (re anything) may not fit all.
Obviously one main focus , IMHO, is Q .Not matter what you build, be it small tank ,loop or long wire or ground run. And at least in my experience with my (long gone) ultra high performance DX crystal sets, the takeaway is save every tiny bit of Q you can. It adds up.
Insulate and insulate again. Radio "loves" air- not solid surfaces. I even used to air stand off ground runs until it reached rod. Maybe overkill to some, but Q bleeding seems to hide in some unexpected places.
In my experience any serious home brew tank/loop,etc, starts with the var cap. It is the first (and maybe the worst) place things can go wrong,IMHO .To me , nothing can replace a 500 pf 4 gang ceramic ( very important that it be ceramic )wired 2 gang as one leg, two gangs as the other. This will eliminate rotor loss. Such caps are often found in old signal generators. If purchased on line, they can be expensive. But WORTH it. I was cheap with mine and enclosed them in airy styrene boxes with ceramic stand off insulators and quick solder terminals. That way I can hard solder them into whatever project I feel like and not have to buy 12 of them (ha, ha). 6:1 (or better) Ball drives and shaft insulation , panel shielding, etc, are musts with theses caps. A definite "investment".
4 gang ceramic did well quite even with solid copper #20 cotton covered wire coated with poly. Such coils with bakelite var caps were noticeably down , even just by ear.4 gang ceramics with 660/46 litz are simply stunning.
Back in the day, I took one of my 660/46 ferrite tank coils off the xtal set and coupled the long wire to the Panasonic's ferrite.
With no ground wire on anything, the results were incredible. Station audio was pinched up really tight and you would swear you were listening to a regen.The selectivity was awesome. Using a ground knocked things down a lot.
I guess any air loop , tank , external ferrite bar coupler , etc are only as good as the components and insulation and the good practices/ techniques used.
I gotta try another BIG Fe external coupler some time.I had SO much of this stuff before the "Great Purge" of 2019. I regret tossing a fair amount of these items (some of which which I MAY reacquire, maybe ) , but the situation dictated it then .Oh well.
Also re air loops used on 50 ohm receivers : I have always been a bit concerned (maybe unfounded) that it has loss written on it (Unless you are talking Kiwa high dollar /50 ohm designed/ regen loops. I had a friend who had one.The performance would blow you away).
I think it would have to be done right to (maybe) get it to work well . Just having a ratio'ed secondary wound right on top of the tuned primary may not be so hot . My understanding , is that the pick up winding may also need to be properly spaced away from the tuned primary as well as ratio'ed. On the AN -200 or a Select -a -Tenna with a jack, this isn't practical due to space considerations . Just too big for a "grab and play" type of store bought antenna.
Even simply tapping an air loop way down for 50 ohms may cause a (serious ?) Q reduction re loading the loop ( comments ?). Admittedly, I have not tried this, so maybe I am not the guy to comment.
I'd say knowledge of RF physics is at a "low to moderate level " (at best) , so if I am off base on anything , I welcome correction for both myself and for the group to learn from .
de NQC
Recent Posts
) and for the superb musical ride across America. Hmmmm.......maybe cities, European countries, or fictional locations next time?
