From owner-qrp-l@Lehigh.EDU Mon Jun 15 01:40:17 1998 Received: from fidoii.CC.lehigh.EDU (fidoii.CC.lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.8.5/8.8.5) with ESMTP id BAA11697 for ; Mon, 15 Jun 1998 01:40:16 -0400 (EDT) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.Lehigh.EDU with SMTP id <13478-60782>; Mon, 15 Jun 1998 01:39:58 -0400 Received: from sunburst.usd.edu ([192.55.228.48]) by fidoii.cc.Lehigh.EDU with SMTP id <12530-44402>; Mon, 15 Jun 1998 01:38:38 -0400 Received: from ppp51.usd.edu (ppp51.usd.edu [192.55.228.184]) by sunburst.usd.edu (8.6.12/8.6.12) with SMTP id AAA09993 for ; Mon, 15 Jun 1998 00:38:37 -0500 Message-Id: <199806150538.AAA09993@sunburst.usd.edu> Date: Mon, 15 Jun 1998 00:38:37 -0500 Reply-To: aweiss@usd.edu Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: aweiss@usd.edu (A. Weiss) To: "Low Power Amateur Radio Discussion" Subject: RE: Coax / Dipole Thread X-To: qrp-l@fidoii.CC.lehigh.EDU X-Mailer: Mozilla/2.1 (compatible; Opera/2.12; Windows 3.1) X-Listprocessor-Version: 8.1 beta -- ListProcessor(tm) by CREN Status: RO Hi gang: I caught the coax dipole vs. balanced feedlines but didn't have chance to reply. Basically, it is a matter of trade-offs. Portable operation is a lot easier with a pre-packaged dipole/coax. Personally, I've found that it is no easier than carrying along 50ft of low-loss twinlead. The notion that a coax-fed dipole never needs a tuner is falacious. The basics: modern rigs, including all QRP designs that I know of, use a fixed output impedance filter circuit designed for 50-Ohms. In many cases, a mismatch between rig and feedline will have minimal undesireable effects. However, our QRP rigs don't tolerate too much of a mismatch for too terribly long. But smoking the final isn't the only concern. A mismatch can invite all kinds of undesireable behavior -- parasitics, harmonics etc. Personally, I'd rather have a tuner along so the SWR can be dropped to zilch even when I'm using coax. But I have done it without a tuner as much as anyone else (or so). Cut to freq. the coax dipole ought to exhibit a reasonable SWR, especially if it is below about 0.2-wavelength because below that point ground loss acts just the same as paralleling a resistor across the antenna feed terminals. Good for keeping down the SWR to around 50-Ohms. I always take a tuner because most of the time I end up settling for a random wire out the motel window to some tree or lightpost. I envy QRP'rs who find motels with rooms up a halfwave and trees or posts in the right place for stringing a dipole. Now, I really got excited when I dropping the xyl off at a motel near the Omaha airport -- 4th floor balcony, BIG trees across the parking lot -- man! I never got to stay in a place like that myself! But it would have been perfect for a dipole, or double-extended Zepp, or horizontal loop, or 3-walength Vee etc.etc. Luck of the draw. Main thing to remember: The basic principle of physics: put a half-wave of wire in the sky and it will radiate equally well regardless of feedline. The only issues are: (1) the amount of power feeding into the antenna after the trips back and forth along the feedline, and (2) effect on radiation pattern, which no one can really take seriously when operating with a simple dipole as opposed to a multi-element gain antenna. Nobody uses a dipole for its radiation pattern -- right? Those of us who favor balanced feedlines thru an efficient tuner are trimming losses. That simple. It is sort of a purist approach (or it used to be!), but then again, it must not be too purist, because "purist" means only the most dedicated fanatics do it. The theory here: at the noise level, 0.75dB can make a difference -- square law detector phenomenon -- on being copied or not. The super-sensitive modern receivers out there trying to copy us are capable of scooping weak signals off the noise-floor. When I do an EZNEC run and see SWR = 3.7 and more (which is what you'll get with a lot of balanced feedline cases), I pull up AC6LA's ZIZL program and see what the diff between coax and balanced feedline amounts to, given RG213 (which I don't have) and low-loss TV twinlead (which I do have). Shaving that .75dB of SWR loss is good enough reason for me to use the cheap twinlead. I think most QRP DX'rs will agree with me on this one -- we NEED every bit of db that we can get when conditions aren't soo hot. Even so, I worked FO0MAC in the pile-up on 30m the other night using the SST at 1.5w, split-freq (yep, swing the knob up, swing it back down etc. -- no RIT) and gawdawful RG8x with its 1.1dB/100ft at 10Mhz loss figure. Half a db gone right there because the SWR is about zilch. Why? Because I bought one of Vern Wright's modified SLVs at Dayton, then modified it a bit more, and on my way out of the fleamarket on Sunday, figured I didn't have anything to feed it with, so I plunked down my $20 for a 50-ft hunk of RG8X so I could operate with the SLV. Actually, I'm thinking of feeding it with twinlead now that I'm back to the ranch. That might get me around +.3dB advantage. Oh well -- if you think about this stuff too much, you lose sight of reality. Like those guys on the stock market tossing around 10,000 shares of this or that everyday. Now, that's something to think about. What's with us QRP'rs, thinking about 0.3dB when we should be thinking about 13000 shares of something up 12 points.... I guess it boils down to the QRP ideal -- knowledge and skill offset the power disadvantage. Depends on what you want out of QRP. 73, Ade