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antenna

terry hughes All Messages By This Member #28925 this topic would be for any tx but i have a bitx40 i set up a 40 m dipole according to the calculated length ie (150x0.98)/7.1 but the swr at the low freq end was bad and at the high freq end it was terrible so i chopped a bit off which improved the swr. I have now got a good swr from 7.0 to 7.2 so why is it now much shorter than the formula length. maybe because it is only about 8 feet above ground any antenna experts out there know the answer terry GM4DSO
John P #28928 I'm not an expert, although I play one on the internet! My guess is that it has to do with the height above ground. I'm having a similar problem trying to tune a hamstick dipole in my attic. Unfortunately the attic floor is insulated with foil backed insulation, and no matter what length I make the antenna, it only wants to resonate at 6.0 MHz. Changing the length does not significantly change the resonance point, but does change the SWR dramatically. -- John - WA2FZW
Paul Artman All Messages By This Member #28929 I am not an expert either, but isn't there a distinction between physical lenth and electrical/electronic lenth?
Michael Hagen All Messages By This Member #28930 开云体育 You can use an antenna modeling program, I think everything matters.? Height and Slope. You might find a free version of NEC or something? Mike, WA6ISP On 6/27/2017 3:02 PM, Paul Artman via Groups.Io wrote: toggle quoted message Show quoted text I am not an expert either, but isn't there a distinction between physical lenth and electrical/electronic lenth?
OZ9AEW #28945 7,1 M is close my Calc. gives 7.1408 M ! but as you write you have placed it 8 feet above ground and there is your problem... expect I expect each leg is about 10% to long due to L/C coupling from antenna and ground ! ps. my antenna is place in only 3 feet above ground to know that problem to good
Lawrence Galea All Messages By This Member #28946 The nearer an antenna is to ground the higher its capacity between the ends through the ground. Therefore, it needs to be shorter. The losses will also be greater since the ground is not a perfect conductor and has losses which not only depend on the type of ground but also on whether it is dry, wet etc. Just to explain it simply with this diagram I put two capacitors from the end to ground but capacity is all along the antenna with higher losses occurring at the end due to high voltage. _________________ _______________ Your dipole _I_ ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?_I_ _ _ ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?_ _ ?Capacity to ground ? I ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?I -------------------------------------------------------- Ground It is easy to see that the higher the antenna is above ground the lower will be its capacity to ground, the lower will be the loses, the lower will be the angle of radiation, the longer will be its length, less changes with the weather etc. However, if you are after NVIS contacts, the antenna will fire virtually up giving you contacts without or with much reduced skip. toggle quoted message Show quoted text On Wed, Jun 28, 2017 at 9:26 AM, OZ9AEW <madsen1960@...> wrote: 7,1 M is close my Calc. gives 7.1408 M ! but as you write you have placed it 8 feet above ground and there is your problem... expect I expect each leg is about 10% to long due to L/C coupling from antenna and ground ! ps. my antenna is place in only 3 feet above ground to know that problem to good
Baruch Atta #28948 Check these possible reasons for shortening the dipole from the"calculated" length.? 1. height above ground 2. nearby metal objects 3. "End Effect"? On Tue, Jun 27, 2017 at 4:47 PM, terryhugheskirkcudbright via Groups.Io <terryhugheskirkcudbright@...> wrote: toggle quoted message Show quoted text this topic would be for any tx but i have a bitx40 i set up a 40 m dipole according to the calculated length ie (150x0.98)/7.1 but the swr at the low freq end was bad and at the high freq end it was terrible so i chopped a bit off which improved the swr. I have now got a good swr from 7.0 to 7.2 so why is it now much shorter than the formula length. maybe because it is only about 8 feet above ground any antenna experts out there know the answer terry GM4DSO -- 73, Joe W3TTT
Andrew Krause All Messages By This Member #28950 Antenna's are a science sufficiently advanced enough to be confused with magic. I say this as someone old enough to remember bunny ear antennas, and having to add small patches of aluminum foil to the antenna, then fiddle with the outer-ring of the channel selection knob so I could watch cartoons static free.? Andrew Krause General Class Amateur Radio License: KM4ZJO andrew.krause@... (404) 732-2369 toggle quoted message Show quoted text On Wed, Jun 28, 2017 at 8:23 AM, Baruch Atta <baruchatta@...> wrote: Check these possible reasons for shortening the dipole from the"calculated" length.? 1. height above ground 2. nearby metal objects 3. "End Effect"? On Tue, Jun 27, 2017 at 4:47 PM, terryhugheskirkcudbright via Groups.Io <terryhugheskirkcudbright=[email protected]> wrote: this topic would be for any tx but i have a bitx40 i set up a 40 m dipole according to the calculated length ie (150x0.98)/7.1 but the swr at the low freq end was bad and at the high freq end it was terrible so i chopped a bit off which improved the swr. I have now got a good swr from 7.0 to 7.2 so why is it now much shorter than the formula length. maybe because it is only about 8 feet above ground any antenna experts out there know the answer terry GM4DSO -- 73, Joe W3TTT
Joe Puma #28953 开云体育 Yes it needs to be at the proper height above ground. Also what type of wire did you use if the wire has insulation there is a velocity factor you must consider. ? Sent from my iPad toggle quoted message Show quoted text On Jun 27, 2017, at 4:47 PM, terryhugheskirkcudbright via Groups.Io <terryhugheskirkcudbright@...> wrote: this topic would be for any tx but i have a bitx40 i set up a 40 m dipole according to the calculated length ie (150x0.98)/7.1 but the swr at the low freq end was bad and at the high freq end it was terrible so i chopped a bit off which improved the swr. I have now got a good swr from 7.0 to 7.2 so why is it now much shorter than the formula length. maybe because it is only about 8 feet above ground any antenna experts out there know the answer terry GM4DSO
[email protected] #28954 1/4 wave dipole 234 / 7,100 is 32ft 9 each leg coxal fed on 30ft fishing pole set as a inverted vee will work well portable and will not need tuner? toggle quoted message Show quoted text On 28 Jun 2017 16:14, "Joe Puma" <kd2nfc@...> wrote: Yes it needs to be at the proper height above ground. Also what type of wire did you use if the wire has insulation there is a velocity factor you must consider. ? Sent from my iPad On Jun 27, 2017, at 4:47 PM, terryhugheskirkcudbright via Groups.Io <terryhugheskirkcudbright=[email protected]> wrote: this topic would be for any tx but i have a bitx40 i set up a 40 m dipole according to the calculated length ie (150x0.98)/7.1 but the swr at the low freq end was bad and at the high freq end it was terrible so i chopped a bit off which improved the swr. I have now got a good swr from 7.0 to 7.2 so why is it now much shorter than the formula length. maybe because it is only about 8 feet above ground any antenna experts out there know the answer terry GM4DSO

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