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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