Return-Path: Received: (qmail 29172 invoked from network); 13 May 2001 20:12:30 -0000 Received: from unknown (HELO murphys-inbound.servers.plus.net) (212.159.14.225) by excalibur.plus.net with SMTP; 13 May 2001 20:12:30 -0000 Received: (qmail 23046 invoked from network); 13 May 2001 20:11:53 -0000 Received: from unknown (HELO post.thorcom.com) (212.172.148.70) by murphys with SMTP; 13 May 2001 20:11:53 -0000 Received: from majordom by post.thorcom.com with local (Exim 3.16 #2) id 14z27T-0005We-00 for rsgb_lf_group-outgoing@blacksheep.org; Sun, 13 May 2001 21:05:35 +0100 Received: from mta5-rme.xtra.co.nz ([203.96.92.17]) by post.thorcom.com with esmtp (Exim 3.16 #2) id 14z27R-0005WZ-00 for rsgb_lf_group@blacksheep.org; Sun, 13 May 2001 21:05:33 +0100 Received: from xtr743187 ([202.27.181.54]) by mta5-rme.xtra.co.nz with SMTP id <20010513200430.HGMO2165018.mta5-rme.xtra.co.nz@xtr743187> for ; Mon, 14 May 2001 08:04:30 +1200 Message-ID: <001c01c0dbe8$33b0e320$36b51bca@xtr743187> From: "Vernall" To: rsgb_lf_group@blacksheep.org References: <3.0.1.16.20010510084513.2c6f9b82@mail.cc.kuleuven.ac.be> <004f01c0d9a2$3334c6c0$2cb21bca@rvernall> <002c01c0db96$045db000$fa8274d5@w8k3f0> Subject: LF: Re: Radiation from loading coil causing difference in current at top and bottom? Date: Mon, 14 May 2001 08:05:38 +1200 MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1; format=flowed Content-Transfer-Encoding: 8bit X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 5.00.2919.6600 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Sender: Dick PA0SE and others, Thanks for trying out the dual trace scope method. You did not comment on the phase at the hot end of the coil compared to the cold end. > Bob Vernall wrote: > > Finding out the phase information of interest does need a dual trace > > oscilloscope or a vector voltmeter. > > I followed up Bob's suggestion using a dial trace oscilloscope. snip > Thus as far as can be judged from oscilloscope traces voltage and current > at the bottom end of the coil were in phase. If there is an extra capacitive > current flowing from the coil to surrounding objects it must be too small > to > show up in this way. This is also confirmed by my earlier experiment which > showed that moving the coil upwards and sideways in the shack made > no difference at all in the currents at top and bottom of the coil. The current and voltage at the cold end of the coil can be arranged to be in phase by a certain tuning of inductance. Using the vector currents in my suggestion, when the antenna is tuned for resistive input (the situation Dick reported on) then for input currents I+Q, that particular tuning introduces a current -Q, so the effective current is then I at the cold end of the coil. There is still no information on the possible phase difference between currents at the cold and hot ends. Using a scope probe to do the hot end test needs a very low power test signal, even a signal generator, so as to not exceed the voltage rating of the probe. I can give this warning, as I fried a nice 10:1 probe when I forgot to remove it after QRP testing, and it also fried the input attenuator to the scope :o( Using the idea of I+Q current going into the cold end of the coil, I suggest there are two tuning conditions of interest: - when the antenna is tuned for maximum current out the hot end (into the antenna wire), for maximum radiation, then the condition at the cold end is not purely resistive - when the loading coil is tuned so the current at the cold end is resistive, the radiation current (hot end) may not be peaked. > So my conclusion is that the difference in current must be due to radiation > from the coil. The late Klaas Spaargaren, PA0KSB, once told me that in > an article on mobile antennas it was stated that a coil of L metre length > produced a radiation equal to a straight wire of that same length. I am not in disagreement with that, for an unshielded coil (as they generally are for mobile whips). If the coil was in a shielded enclosure then it does not have a far field to consider, so could not be allocated an effective height component. For unshielded loading coils, the height of the coil should be included in the overall antenna system. For a top loaded vertical, the difference in current from ground level to the node where top loading is (horizontally) connected, the reduction in current with height is far less than for a pure vertical (with no top loading), however I agree that it is some value and contributes to the observation that less current comes out the hot end than goes in the cold end. However, I believe there is additionally an I+Q situation as the stray capacitance to ground for a loading coil is far more than when the coil is substituted by a straight wire for considering the radiation current distribution. 73, Bob ZL2CA