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: [email protected] [mailto:[email protected]] On Behalf Of Phil Sent: Friday, August 22, 2008 8:18 PM To: [email protected] 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?

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" [email protected] To: [email protected] 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 [email protected]. 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: [email protected] To: [email protected] 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 [email protected]. 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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