ARLA/CLUSTER: Antenas SOTA - "Linked Dipole" para HF

[Joel Lobão]

Olá Pedro,

por acaso não concordo que se devem usar antenas pouco eficientes no QRP.
Até pelo contrário. Acho que devem usar antenas o mais eficientes possivel
para compensar a pouca potência.

Para a semana vou tentar colocar aqui umas fotos de uma loop magnética
construida pelo meu grande amigo António Fernandes CT1FJL. Falta testá-la
em campo aberto.

73 de CT1HXB - Joel Lobão

No dia 10 de Abril de 2012 16:24, Pedro Correia <
pedrocorreia1111  hotmail.com> escreveu:

>  Boa tarde,
>
> Obrigado pela partilha. Este tipo de antena é provavelmente o mais usado
> em /P, no ano passado o CT-QRP-CLUB fez uma pequena série de kit's
> semelhantes a esta antena (faz os 20 M e 40 M) que distribuiu pelos sócios.
> Não posso é deixar de realçar a coragem deste colega ao dizer: "For the
> majority of UK hams at least the G5RV is as adventurous as it gets. Try to
> tell them their antenna is a poor choice and you might as well be an
> atheist preaching from the pulpit to a church full of Southern Baptists.
> For home use even something as simple as an Off Centre fed dipole could
> offer a better bet than the a G5RV in a similar space, but that is a
> discussion for another time."
> Será que uma antena OCF não é menos complicada e até mais leve se
> construirmos o transformador 1:4 para níveis de potencia baixos? O único
> problema é que se calhar não se pode eliminar o sintonizador de antena...
> Se uma antena é sempre uma solução de compromisso, então com a variável
> peso a pesar muito ainda complica mais.
> Em tom de brincadeira: QRP com uma yagi monobanda de 7 ou 8 elementos é
> batota, por isso nada de antenas muitos eficientes!
>
> 73
>
> Pedro CT7AEZ
>
>
> ------------------------------
> Date: Tue, 10 Apr 2012 08:09:49 +0100
> From: ct1fbf  gmail.com
> To: cluster  radio-amador.net
> Subject: ARLA/CLUSTER: Antenas SOTA - "Linked Dipole" para HF
>
>
> Linked Dipole for Portable Operations Such as SOTA
>  One of the things those of us involved in amateur radio for a number of
> years tend to forget is that things we have learned over time are not
> necessarily common knowledge even among our peers. So when I am operating
> HF portable and I tell people I am running a linked dipole I expect them to
> know what I mean. The truth is rather sad in that more often than not they
> do not have a clue. For the majority of UK hams at least the G5RV is as
> adventurous as it gets. Try to tell them their antenna is a poor choice and
> you might as well be an atheist preaching from the pulpit to a church full
> of Southern Baptists. For home use even something as simple as an Off
> Centre fed dipole could offer a better bet than the a G5RV in a similar
> space, but that is a discussion for another time.
> For now I want to talk about the linked dipole as used by numerous SOTA
> operators for two reasons. One: several people have asked me to write
> something and Two: an Internet search for linked dipoles did not come up
> with anything useful. The main advantage of a linked dipole is there is no
> requirement for an ATU (antenna tuning unit) so we have one less heavy item
> to carry in our back-packs. Because on SOTA we as the activators choose the
> frequencies we work on we can optimise our antennas for the very
> frequencies we operate on. So on 10 metres I can normally be found around
> 28.5MHz and so I started my linked dipole by cutting a half wave dipole for
> about 28MHz and then using an antenna analyser or the Yaesu FT-817 and an
> SWR bridge I fold back the ends until the lowest dip in SWR (or max field
> strength if you have facility to measure it) is at 28.5MHz.
> Note: Fold back. Do not trim until you have determined where to cut. A lot
> of amateurs trim a 1/4 inch a time until the SWR is acceptable, but that
> way you end up with an antenna that has had either too much or too little
> lobbed off.
> Next I add an insulator as seen above and a link. I have Used Anderson
> Power Pole and automotive type bullet connectors. I would also recommend
> the small gold bullet connectors used by the radio control model community
> and bought cheap in bulk on eBay. Power Poles have the advantage that they
> are easy to deal with with cold fingers or in gloves. Then I add the
> section of wire required to make the total length suitable for 20 metres
> and tune in the same way and so on.
>  <http://www.flickr.com/photos/john_clifford/535686398/>The pictures are
> of a linked dipole made by John GW4BVE and available on Flickr. I made my
> linked dipole independently of John and when I did a joint activation with
> him was surprised at the similarities of his design and mine. The
> differences are I use a commercial dipole centre, I don’t bother with a
> balun and I use RG-58 coax. I have actually never seen RG174 coax or plugs
> for sale other than mail order, which is the main reason I don’t use it. It
> would be much lighter to carry of course. The main surprise I got was when
> I measured my dipole elements they matched John’s sizes in the above
> picture exactly. However if you are using a thicker wire than we used or
> tuning for the CW end of the bands your lengths could vary a little from
> the sizes shown. I have since constructed a second dipole blind using
> John’s measurements and when I checked later with an antenna analyser it
> needed no adjustment.
> For deployment in the field the antenna can be strung from handy trees,
> but as on most hills trees are not an option then a telescopic 6 to 10metre
> fishing pole (also known as a Roach Pole) is used as a mast. The antenna is
> deployed in an inverted vee formation.
> The fishing pole mast is held up by guys. I use a collar made from PVC
> which fits just above the bottom section of the fishing pole with three
> short guys and use heavy duty rock pegs, that look like huge masonry nails
> to hold the ends. Other activators guy the mast near the top, while some
> use a single guy and let the antenna act as the other two guys. The choice
> is yours.
> The reason I guy at the bottom is that it keeps the guys short and easier
> to manage. I do not like the single guy method as it adds additional strain
> on the antenna. Easy of management is the reason for the next image.
> Winders can be made from ply as John shows here or made from plastic card
> (available from model shops) like mine or purchased from kite suppliers,
> eBay or from Richard G3CWI’s SOTApole website.
>  <http://www.flickr.com/photos/john_clifford/535686398/>
> The ends of the dipole as shown in John’s images have an insulator and
> then a length of thin guy rope. When deploying the dipole it is a good idea
> to get the ends as high off the ground as possible. Tests have shown that
> the difference between having the ends of the dipole low to the ground and
> at a mere three feet (1m) off the ground can be considerable. As a result
> of these tests I now raise the ends of the dipole by using my walking
> poles, extended to their full length, to hold the guys on the dipole ends
> just before they pegged. To simplify this a Y shaped double guy can be
> fitted to simplify the support. Because I rarely activate alone I use a
> single guy and a pair of walking poles on a cross formation to do the same
> thing.
>  <http://www.flickr.com/photos/john_clifford/535785237/>
> This last image shows John’s dipole centre made from a disc of perspex
> with a hole for using the single guy method of supporting the dipole and a
> choke balun. As I said I use a commercial dipole centre and although the
> choke balun is a good idea I have never noticed any RF feedback running
> even 100 watts without one.
> My thanks to John GW4BVE for use of his images. John says:
> *I have been asked to publish some photos of my /P HF antenna. It is a
> dipole with the element lengths adjustable by plugging/unplugging
> connectors. The antenna weighs 580grams including feeder and winders. The
> wire is surplus thin connecting wire and the feeder is RG316 (PTFE RG174).
> Insulators are small pieces of scrap plastic. It is normally erected
> inverted V fashion with the centre supported by a 7 metre fibreglass
> fishing pole and the ends by my walking poles. This antenna covers 80m,
> 60m, 40m, 20 & 10m, but it can be made for any combination you chose. No
> ATU is required. The winders are made from Lightply, which is sold by good
> model shops.*
> *The drawing shows approximate dimensions of my HF dipole. The balun has
> been omitted for simplicity and I now use RG316 feeder which I find is
> higher quality and of course has less loss, although as the feeder is so
> short the loss at HF is negligible. Please note that you should tune the
> antenna at the height you are going to use it on the hills. Make each
> section slightly longer and cut to minimum SWR starting at the highest
> frequency and working to the lowest. You will not get full coverage on 80m
> so it it is best cut for your preferred operational frequency.*
> *The balun could be omitted to make the antenna lighter. The balun core
> is 3.5cm diameter and is made of type 77 ferrite with about 20 turns of
> coax.*
> *The insulators ensure that there is no strain on the connector itself.
> The connectors are Powerpoles and are available from CPC and Farnell in the
> UK or can be ordered from the USA. Of course any reliable single pole
> connector could be substituted.*
> Note: Click on the images to take you to John’s images on Flickr where
> they can be seen full size.
> Fonte: Blogue de GW7AAV (
> http://cqhq.wordpress.com/2011/03/19/linked-dipole-for-portable-operations-such-as-sota/
>  )
>
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