John G3PQA wrote:
>
> From: "john" <jb@beaumonts.karoo.co.uk>
>
>> Could anyone advise me about top loading when 4 wires are use as an
>> alternative to 2 with this type of antenna. My present antenna is
>> 82ft vertical with 4 top lading wires each 35ft long, I have been
>> told only 1 wires counts as the loading/lengthening on this type of
>> antenna which I dont fully agree/understand. Is it not that a 2 wire
>> tee you calculate the missing portion of your vertical lets say I
>> needed 127 ft for 1.83mhz and my vert is 82ft so 45ft too short, so
>> lets say 60% of 45ft = 27ft I would need 2 top wires 27ft or maybe
>> 70% of 45ft = 2 x 31.5 ft.
>
>
> According to EZNEC a tophat of 4 x 35ft wires gives resonance on about
> 1.6MHz , assuming single wire vert.
> If the wires are shortened to 22ft resonance is 1830kHz
> John
Thanks for the EZNEC model John. Are the wires in your model sloping
down or straight as a 45 degree slope would tend to make them appear a
bit shorter?.
I think the match on 1.8mhz may be easier with the lower frequency
resonance on 1.6 Mhz as the impedance is driven up and the tuning to 1:1
maybe possible with a just some series capacitance. I think a resonant
antenna with a good ground would be down about 20 to 25 ohms or so but
the longer one closer to 40-50 ohms and near perfect if the inductive
reactance is tuned out.This would IMHO remove the need for an L network
which might be easier to handle for some. .
Many hams share the myth that resonance is required on a Marconi for
peak performance...not so.... as it is all in the match. In my view if
the match is simple and easy then the antenna is just as good being a
bit longer on the operating frequency. Plus with a motor driven
variable you can cover the whole band most of the time rather than being
limited to 30 Khz segments. At some point..depending on the top loading
the antenna may even be usable as a 3/8 wave or close to it on 3.5 Mhz
just straight without the capacitor in series.
73,
Herb Schoenbohm, KV4FZ
> ------------------------------------------------------------------------
>
>
> No virus found in this outgoing message.
> Checked by AVG - www.avg.com
> Version: 8.5.431 / Virus Database: 270.14.124/2598 - Release Date: 01/03/10
> 09:41:00
>
> ------------------------------------------------------------------------
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
|