[amsat-bb] Re: Determining the length of a driven element
ko6th_greg at hotmail.com
Sun Aug 24 20:39:41 PDT 2008
...except, of course, that I'm assuming it's lower than that in the middle.... That would be an interesting measurement to make. If you're on a glide slope 5:1 down to 4:1 going towards the higher end of the band, then I agree the antenna is too long. But assuming (here I go again!) that the original length was something reasonably close to 468/f in feet, then you've probably got something else going on to set it out of tune. Check for loose connections, nearby metal things, bad insulators, etc. before doing any pruning.
> From: ko6th_greg at hotmail.com
> To: phillor at telstra.com; amsat-bb at amsat.org
> Date: Sun, 24 Aug 2008 20:06:18 -0700
> Subject: [amsat-bb] Re: Determining the length of a driven element
> Maybe I'm missing something, but I think there's a more fundamental issue here. If the SWR is way out of range at both ends of the band, it would appear to me that the band is too wide for the antenna. No pruning of the antenna is going to fix both ends. You need to pick a center frequency for where you want to operate, and adjust the antenna for that.
> Greg KO6TH
>> From: phillor at telstra.com
>> To: amsat-bb at amsat.org
>> Date: Sat, 23 Aug 2008 13:18:21 +1000
>> Subject: [amsat-bb] Determining the length of a driven element
>> During a casual conversation recently, the topic of aerial trimming came up.
>> Perhaps this may be a little off-topic here but since I don't know the answer
>> I thought this would be a good place to ask anyway.
>> Say a dipole has an SWR of 5:1 at one end of a band and 4:1 at the other end.
>> Further, let's say that the dipole will be used at the centre of the band and
>> with a reasonable SWR. It has been determined that the dipole needs to be
>> shortened. The question is by how much?
>> The person asking the question is reluctant to pull the aerial down a dozen
>> times so that short lengths can be nibbled off each end and instead wants to
>> know if the amount that needs to be cut off can be determined from the
>> difference in the SWR at both ends of the band.
>> A logical question I suppose but is there a reasonable answer?
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