[Antennas] Single feed on 5 band quad (correction to my previous post)
John Tait
[email protected]
Mon, 28 Oct 2002 18:02:37 -0000
Subject: Re: [Antennas] Single feed on 5 band quad (correction to my
previous post)
Thanks for the interesting thread folks...
I agree with everything as posted below.. Just to bring out
Joe's point about 1/4 w/l coax links a little further. These 1/4 w/l links
are used to act as switches, and NOT as impedance matching devices as per
the usual 75 Ohm 1/4 w/l sections. LB's modelling of my spider Quad predicts
40 Ohm minimum, to 80 Ohm maximum feed impedances. My measured values agree
within a few Ohms. Therefore, the match to 50 Ohm coax varies from excellent
to acceptable (at worst).
If you MUST use a common feed system, then the "least bad choice" is
to use two different feeds, and group 20, 15 and 12m on one feed, and 17 and
10m on the other.
As well as the trove of information at Cebik.com, LB also has written two
excellent books on Quads. These are available from Antennex.
73
John EI7BA http://www.iol.ie/~bravo/
> Dear Ron and Jaap:
>
> I have pasted below personal correspondence from W4RNL regarding the
> issue of separate vs common feeds for multiband quads. My modeling and
> on-the-air experiences generally confirm his modeling results. I
> strongly recommend separate feeds and the use of 1/4 wavelength coax
> links between each coax terminal on the remote switch box and the
> corresponding loop feed point. The switchbox end of each link should be
> left open. This step effectively closes the
> inactive driven loops as Ron has recommended. You will have fewer band
> interaction problems and performance that is up to the generally
> accepted standard for quads. There is a lot of information on this
> subject at W4RNL's web site.
>
> 73, Joe
>
> Joe,
>
> I have done extensive computer studies of common-feed vs. separate feed
> quads dating back to when MININEC was the only available modeling program
> for the PC. There will be a detailed study of the question, using NEC-2
> and NEC-4 (although for wire quad loops, all of the same wire diameter,
> NEC-2 is perfectly good), in likely the Fall issue of Communications
> Quarterly. (They have had the article for about a year, but had some
> other stuff of mine to print first.) Lew used some of my early MININEC
> work in his book.
>
> Essentially, my conclusions are similar to yours. I modeled a 5-band quad
> of spider design (constant spacing in terms of fraction of a wl, not in
> feet). Common feed distorted 10 meters, and the problem came from 2
> sources: 20 meters and 12 meters. The 12-meter DE on this 2-el per band
> model is close to being a 10-meter refelctor surrounding the 10-meter DE.
> The impedance on 20 when driven at 10-meters is so low that it takes up to
> 20-25% of the power, with a pattern that is essentially side lobes (with a
> 2 wl loop at 10) and seriously distorts the 10-meter pattern on its own.
> Ground reflection can restore some of the distorted pattern into usefully
> directed radiation, but the result is far from a normal 2-el quad pattern.
> Without some standard of comparison, most operators cannot detect the
> degradation of performance produced by common feed.
>
> When the loops for the unused bands are shorted--and the loops optimized
> for each band in the 5-band model--performance is up to the best a 2-el
> quad can offer on all bands, with gain peaking at about 7 dBi FS gain, and
> very good F-B. With a constant fraction-of-a-wl spacing (I settled on
> about 0.16 wl for the best compromise among gain, F-B, and Feed Z), the
> outer (20-m) quad showed a slightly higher feed Z and the inner quad
> (10-m) showed a slightly lower feed Z, compared to the interior quads, all
> of which settled in at about the same value.
>
> Based on this work, I have always recommended separate feed--often 1/4 wl
> sections open when not in use to yield a short at the wires to keep the
> loops closed unless actually being fed. A remote switch at the hub is one
> of the better means of being able to use only one feedline down the tower.
> However, like the rotator, it needs periodic maintenance checks.
>
> I have never seen any modeling outputs from the makers of common-feed
> quads: GEM, Cubex, Lightning Bolt; only verbal assurances without an
> explanation of their support. On the other hand, Antenna Mart (Bill Wall)
> has always advocated separate feed, and one can transform Cubex quads
> easily into separate feed quads, although one might want to model the
> dimensions. A friend in Belgium has a 3-element Cubex quad that he
> modeled and modified for max performance and made significant
> improvements. He used AO for his modeling, and I confirmed his
> preliminary work with some 600-segment NEC-4 models.
>
> I have also heard, although indirectly, that one quad maker claims the
> quad cannot be adequately modeled via any of the NEC/MININEC software.
> This claim seems more of an excuse for not re-engineering the product line
> rather than anything that can be substantiated. Quads have no properties
> that stress the modeling software anywhere near its limits. The only real
> caution comes with a common feed model: it requires a straight common
> wire to which all the driven loops join at the ends, and the segmentation
> must be within the length ratio limitations for the adjoining loops
> segments.
>
> It is unlikely that quads will ever be put to a satisfactory range test,
> given their ungainliness compared to multi-band Yagis. Hence, the
> disputation will likely continue. However, anecdotal reports--while
> interesting--do not establish anything in science and engineering. On the
> other hand, the modeling software has (when well-used) proven its
> correlation with reality and therefore is presently a stronger
> confirmation-of-hypothesis technique than operating reports.
>
> I hope these notes are useful to you.
>
> -73-
>
> LB, W4RNL
>
>
>
> Ron W7MRR wrote:
>
> > Hi Jaap:
> >
> > I have spoken with the Owners of both Gem Quad and Cubex and both
> have fed their quads with a single feed line, claiming it worked well.
> They both said that if there is any compromise, it was minimal.
> >
> > After reading all the articles on the subject, consisting of both
> theoretical and actual experiences, I decided to feed my 4 element quad
> (a Gem Quad) with two feed lines. One for 20/15/12 and one for 17/10.
> Looking at harmonics, this seemed to isolate the elements that might
> interact. In addition and if you are going to use a remote switching
> arrangement [to utilize a single coax lead in from the antenna
> location], you should try to make sure your switching arrangement shorts
> the lines that are not in use.
> >
> > Good luck.
> >
> >Ron W7MRR
> >
> >
> >
> >
> > --- On Thu 10/24, Jaap de Jonge wrote:
> >From: Jaap de Jonge [mailto: [email protected]]
> >To: [email protected]
> >Date: Thu, 24 Oct 2002 09:44:02 +0200
> >Subject: [Antennas] Single feed on 5 band quad
> >
> >
> >
> >>Hi all,
> >>
> >>I'm considering building or buying a 5 band HF quad (10 thru 20
meter).
> >>There's a local antenna company, that markets a model that has a
single
> >>feed for all the elements from 10 to 20 meter. You can see a picture
of
> >>it at http://www.gbanttow.nl/GBFGQUADmessung.htm . I'm interested
> in the
> >>following:
> >>1) Isn't the radiation pattern of such an antenna severely
compromised
> >>(side lobes etc.), because of mutual interaction of the radiators?
> >>2) Can anyone tell me from experience if this concept works or not?
> >>3) Has anyone ever modeled such a quad with a single feed using
antenna
> >>modeling software?
> >>
> >>Thanks for the assistance!
> >>
> >>73
> >>Jaap
> >>PA8DX
> >>
> >>
> >>
> >>- - -
> >>
> >>Your moderator for this list is:
> >>Larry Wilson KE1HZ [email protected]
> >>_______________________________________________
> >>Antennas mailing list
> >>[email protected]
> >>http://mailman.qth.net/mailman/listinfo/antennas
> >>
> >>
> >>
> >
> >_______________________________________________
> >Join Excite! - http://www.excite.com
> >The most personalized portal on the Web!
> >
> >
> >--- StripMime Report -- processed MIME parts ---
> >multipart/alternative
> > text/plain (text body -- kept)
> > text/html
> >The reason this message is shown is because the post was in HTML
> >or had an attachment. Attachments are not allowed.
> >Please post in Plain-Text only.---
> >- - -
> >
> >Your moderator for this list is:
> >Larry Wilson KE1HZ [email protected]
> >_______________________________________________
> >Antennas mailing list
> >[email protected]
> >http://mailman.qth.net/mailman/listinfo/antennas
> >
> >
> >
> >
>
>
>
>
>
> - - -
>
> Your moderator for this list is:
> Larry Wilson KE1HZ [email protected]
> _______________________________________________
> Antennas mailing list
> [email protected]
> http://mailman.qth.net/mailman/listinfo/antennas
>
>
>