I've done years of modeling, building, testing and MFJ analyzing, but then
again I am a Rural Sociologist! So here goes my 2-cent's worth:
1) even if the end of the feedline makes your recvr happy at 50 ohms, that
does not mean that there are NOT SWRs at different points along the feedline
and radiating points - Therefore loss of ERP.
2) The quest for an all band antenna is sort of like the quest for the Holy
Grail - lost of effort but you really need to know what it looks like and if
it is resonant at the feed point that only means that the Grail is half
full.
3) The other half to fill the Grail are the lobes, hence modeling is
necessary (EZNEC is my favorite). That means taking into consideration the
height to boot.
4) finally, hang it up and test it on the air. That moment is anologous to
sipping from the Grail.
73,
Kris (KA2OIG)
Merschrod
123 Warren Road
Ithaca, NY 14850
www.merschrod.net
----- Original Message -----
From: "Dr. Gerald N. Johnson" <geraldj@weather.net>
To: <tentec@contesting.com>
Sent: Sunday, February 21, 2010 11:41 AM
Subject: Re: [TenTec] antenna analyzer reading?
> On Sun, 2010-02-21 at 15:00 +0000, John Chance-Read wrote:
>> Denton's query and your answer was timely helpful to me - but it's a
>> little above my head.
>> 1. I have a 80m G5RV with all required dimension met (10 metre mast with
>> ends at 5 metres) and just fits my plot. Using my MFJ 270 analyzer I
>> find most bands are covered but not at 1:1.
>> It uses a 28 feet of 70 ohm ladder line and about 50 feet of 50 coax to
>> the shack.
>
> Then its performing as well as can be expected. Its a compromise and the
> feed line lengths are critical. Yours seem a bit short.
>
>> 2. I also wanted 160m so I simply extended the dipole, zigzagged back
>> down the garden towards the mast - of course it does not match at 80m but
>> surprisingly other bands were OK (if not better) but not 20m.
>
> Now its no longer resembling the G5RV.
>
>> 3. I happend to have a pair of 80m traps. So as to bring back 80m on the
>> same system I fitted the traps at end of 80 dipole section. Unexpected
>> but advantageous, I found my 160m had to be shortened by about 2-3 metres
>> to get resonance.
>
> Your traps only help on 80 meters and the parallel tuned circuit of an
> 80 meter trap on 160 is inductive so acts as a loading coil.
>
>> 4. I have an Auto ATU fitted in my Orion but disable so that I could
>> make SWR measurements. with Orion and the MFJ270. (The ATU can match all
>> but 10m).
>
>
>> 5. I now find I have the following SWR indications in the shack (at the
>> end of the co-ax)
>> 160m - better than 3 : 1 between 1.8MHz and 1.9MHz with a null of
>> 1.5 : 1
>> 80m - resonant at about 3.4MHz but better than 3 : 1 up to 3.7 MHZ
>> 40m - better than 2:1 across band and 1 :1 above 7.2 MHz
>> 30m - not resonant
>> 20m - high vswr
>> 17m - better than 2 : 1 across band
>> 15m - better than 2 : 1 across band
>> 12m - better than 4 : 1 across band
>> 10m - better than 3 : 1 across band
>> 6. All of the above except 30 metres can be matched to 1 : 1 with the
>> aid of the Orion ATU
>
> An external manual tuner may help on some bands. Adding some coax to the
> feedline on the bands it won't tune can help by moving the impedance to
> a region the tuner can handle.
>
>> 7. A knowledgeable friend of mine tells me that the SWR becomes
>> meaningless with the confusion of techniques that I have employed and
>> that I should throw away the traps and the Analyzer and just put up a
>> single long wire antennae with any length open wire to the shack and
>> depend on the ATU.
>>
> I prefer that long wire be center fed with balanced open wire line and
> to use a truly balanced tuner, not a tuner tuning through a balun on the
> antenna side. For many years I had one about 90 meters long center fed
> and I used it from VLF through 2m with an assortment of tuners. And long
> before that I had one only 80 feet long that I used from 160 through 10m
> with a manual link coupled tuner that I could convert from series to
> parallel tuning.
>
> The end fed wire can work effectively, but the feed wire radiates too
> close to the shack giving RF feedback problems and also hears all the
> computers and accessories in the house that the balanced antenna and
> feedline reduces.
>
>> My question : - Is it better to try and achieve the lowest SWR (peferable
>> at the end of the 70 ohm ladder line as this becomes part of the antenna)
>> and then rely on the ATU (as I do) or is my friend correct to go the easy
>> way.
>>
> Both work though its not possible with the G5RV to achieve low SWR on
> all bands, and adding a 160m wire and traps takes it away from being a
> G5RV. Its only a G5RV if the wire, and the two segments of feed line are
> his original dimensions. Changing the dipole length, the balanced feeder
> length and the coax length, its no longer a true G5RV and the impedances
> will be different.
>
> I prefer the center fed wire that reaches from support to support and
> the open wire feeder with balanced tuner with feed line length that
> reaches from the antenna center to the tuner in the hamshack. There will
> be difficult to tune frequencies that the automatic tuner might not
> tune, but some manual tuner will tune. There have been dimensions for
> "preferred" length combinations of wire and feeder published since the
> late 1930s in magazines and handbooks, but they don't consider 15m or
> WARC bands which hadn't been drempt of yet.
>
> Neither scheme will have a radiation pattern consistently broadside to
> the radiator. When the wire is more than 1.3 wavelengths long (center
> fed or end fed) there is no radiation at right angles to the wire.
>
> There are many wire antenna ideas in Pat Hawker's (G3VA) Antenna Topics
> Book available from RSGB including many discussions of the G5RV type
> antenna and its limitations.
>
>> John - G4BOU
>>
>>
> 73, Jerry, K0CQ
>
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