Hello,
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?
Phil, SWR should be less than 2:1 Several computer modeling programs exist that will give you and answer to your question. Length is the resonant part of the SWR, resistance is the other part, either can cause a high VSWR and usually both contribute.
Art, KC6UQH -----Original Message----- From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On Behalf Of Phil Sent: Friday, August 22, 2008 8:18 PM To: amsat-bb@amsat.org Subject: [amsat-bb] Determining the length of a driven element
Hello,
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?
Thank you, I knew I'd get useful responses from this group.
I've been in contact with the owner of the aerial and it seems that he has neither an analyser nor modeling software.
This has got me interested now and so I'll see what software there is available that will run under Linux, lots I suspect.
Nope
Phil wrote:
Hello,
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?
Hi, Phil
If 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 then a good method in the absence of any instrument like an impedance meter is to keep the dipole down in the field and feed it with a coax cable which electrical lenght is exactly 1/2 wavelenght or any odd or any even numbar of 1/2 electrical wavelenghts cut for the wanted working frequency of the dipole.
No matter if the characteristic impedance of the above feed line has or not the same impedance of the dipole because the impedance Z = R+/-jX ohm shown by the dipole will be seen without transformations at the open end of the above coax line so that any adjustement made over the dipole down the field to get the lowest SWR will have the same effect as to work over the roof.
Another advantage to use such 1/2 electrical wavelengts of coax cable down to the shak is the possibility to connect to it a network analyzer and measure the same complex impedance R+/-jX of the antenna like to be directly connected with the instrument to the antenna connector over the tower.
The amount that needs to be cut off or to add cannot be determined from the difference in the SWR at both ends of the band.
In addition the same value of SWR can be generated if the frequency feeding the dipole is too low or too hight with respect to the resonant frequency of the dipole because the SWR meter alone cannot recognize if the reactive part jX of the impedance producing the same value of SWR is inductive +jX or capacitive -jX
If decreasing the frequency with respect to the working frequency the SWR decreases it means that the dipole has inductive reactance at the working frequency and is too long
If increasing the frequency with respect to the working frequency the SWR decreases it means that the dipole has capacitive reactance at the working frequency and is too short.
When the exact lenght of the dipole has been determined and all reactances are cancelled out probably the SWR cannot be reduced anymore because the real part R of the impedance is different from the characteristic impedance of the feed line and at this point a matching system must be used to get Z = R +/-j0 ohm
If the 1/2 wave dipole is open at the center as the dipole in question then a T- match with a suitable balun is suggested.
If the 1/2 wave dipole is not cut at the center then a gamma-match matching system without any balun can be used.
73" de
i8CVS Domenico
----- Original Message ----- From: "Phil" phillor@telstra.com To: amsat-bb@amsat.org Sent: Saturday, August 23, 2008 5:18 AM Subject: [amsat-bb] Determining the length of a driven element
Hello,
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?
-- Regards, Phil _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
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@telstra.com To: amsat-bb@amsat.org Date: Sat, 23 Aug 2008 13:18:21 +1000 Subject: [amsat-bb] Determining the length of a driven element
Hello,
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?
-- Regards, Phil _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
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...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.
Greg KO6TH
----------------------------------------
From: ko6th_greg@hotmail.com To: phillor@telstra.com; amsat-bb@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@telstra.com To: amsat-bb@amsat.org Date: Sat, 23 Aug 2008 13:18:21 +1000 Subject: [amsat-bb] Determining the length of a driven element
Hello,
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?
-- Regards, Phil _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Get ideas on sharing photos from people like you. Find new ways to share. http://www.windowslive.com/explore/photogallery/posts?ocid=TXT_TAGLM_WL_Phot... _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
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participants (5)
-
Art McBride -
Greg D. -
i8cvs -
Joe -
Phil