[TenTec] antenna analyzer reading? and the G%RV

Sun Feb 21 17:56:47 PST 2010

Gee, anyone knows the Grail was not a cup !   LOL

Sorry, won't waste any more bandwidth with my alleged humor :)

73 de Gary, AA2IZ

----- Original Message ----- 
From: "Kris Merschrod" <Kris at merschrod.net>
To: <tentec at contesting.com>
Sent: Sunday, February 21, 2010 11:52 AM
Subject: Re: [TenTec] antenna analyzer reading? and the G%RV

> 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 at weather.net>
> To: <tentec at 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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