Topband: broadbanding an L

[Michael Tope]

----- Original Message ----- 
From: "dennis o'connor" <k8do at mailblocks.com>
> OK, I'll play the bad guy here - IT CAN"T BE DONE, period! And I am 
> going to challenge those who claim it can be done, to "fish or cut 
> bait." More on that in a minute..
> 

> 
> After you have overheated your brain, and fried EzNec, trying to do 
> that and give up, then challenge #2 is to do the same, but I will allow 
> a single, fixed network at the feed point... One fixed coil and one 
> fixed capacitor (no variable elements, no multiple paralleled caps, 
> coils. etc., no mechanical or electrical switching per the original 
> request for help)

 I have an EZNEC/XZIZL model of an 80 meter dipole that
will cover the entire 80 meter band (3.5 to 4.0 MHz) with a 
VSWR under 2.0:1 with very good efficiency. You do need
a fixed LC network (challenge #2, I guess), however, but no 
adjustment is required in order to QSY from one end of the 
band to the other. The LC network is a lumped version of 
the coax transformer broadbanding technique that W6NL 
(ex. W6QHS) published in the amateur literature. I believe 
this is essentially what Tom, W8JI was referring to in his 
recent post. I am sure it could be easily adapted to 
broadband an inverted-L. The fundamental tradeoff is in the 
parallel LC network. The required loaded Q of the network 
goes up in rough proportion to how much you want to widen 
the VSWR bandwidth of the antenna. At some point, you 
will run into the limitations of practical lumped components 
(high circulating current and finite inductor unloaded Q), but 
I think this technique would still be practical for getting 10% 
VSWR bandwidth from the 160 meter inverted-L in question 
if one used good quality lumped components. 

As far as challenge #1 goes, I am trying to imagine a bent
(e.g. L ) version of the diskcage vertical that N7DF used to 
use back in the 80's :) 

73 de Mike, W4EF..................