I am not an antenna designer but I can read. G5RV's articles are
available in the ARRL's antenna books. He says that the antenna is a 20
meter antenna that was built exactly. That is, it exactly fit in his
back yard. Now we have all this mystery and expectations that the
antenna will work on all bands well, well at least work on all bands. We
have half size, double size and antennas with traps all claiming to be
G5RV's. I don't think so. G5RV laid out the antenna and then said his
test showed some bands to be troublesome, and he is right. But it does
have 20 M lobes that are useful and mine works into Europe and the West
Coast, just what I wanted. It works on other bands and that is a plus.
It is squirrelly on 15 M, as he stated.
This not to say that there are not other dipoles of various designs and
arrangements will work well, just stop calling them G5RVs. If they
work, then name them as desired.
John Graves
WA1JG
jh.graves@verizon.net
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.
> 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.
> 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.
> 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
> 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.
>
> 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.
>
> John - G4BOU
>
>
>
> ________________________________
> From: Dr. Gerald N. Johnson <geraldj@weather.net>
> To: tentec@contesting.com
> Sent: Sat, 20 February, 2010 7:49:47
> Subject: Re: [TenTec] antenna analyzer reading?
>
> On Sat, 2010-02-20 at 08:46 -0800, Denton wrote:
>> Hi all...a little off the Ten Tec groups, but...
>> I have an MFJ 259B that I use and am a little confused about the R and X
>> readings...
>> I am trying to determine the true losses of my 235 ft horizontal loop, fed
>> with 600 ohm ladderline, then to a 4 to 1 voltage balun outside the shack.
>> SWR seems reasonable..below 3.8 to 1 80 thru 10 meters with the exception of
>> 60 meters.
>
> Most of the R of a decent antenna is radiation resistance, not wire loss
> resistance. You can't tell about an antenna's efficiency or looses by
> looking at the feed impedance. Unless its a very well documented antenna
> like a quarter wave vertical that should show about 35 ohms R if the
> grounds are very good. That's at the antenna, measuring through a feed
> line will confuse you. Many a ham vertical measures more like 60 or 70
> ohms feed R at quarter wave resonance and that's an indication that the
> ground resistance is significant and decreasing the efficiency or
> increasing the energy dissipated in that ground resistance and not
> radiated.
>
>> I run about 15 ft of good quality coax to the shack.
>> I also run 450 ohm feeders to the shack and a johnson matchbox to compare
>> signal levels...and to be honest, I can't tell much difference on transmit
>> nor recieve....except the signal to noise is a bit better with the balun vs
>> the matchbox.
>
> That better S/N may be a sign that the balun's better isolation of
> ground to the balanced side is reducing reception of noise by the
> feedline.
>
>> I have tried 4 to 1 and 1 to 1 current baluns, but the voltage balun gives
>> me better matching.
>
> As one might expect when the main feed line is 450 ohms and the antenna
> tends to be higher impedance than low impedance. Those ratios are
> impedance transformation ratios. 4:1 with a 200 ohm load shows 50 ohms
> to the transmitter (or antenna bridge) if the 200 ohm load is on the
> high impedance side. 1:1 doesn't change the impedance if its perfect,
> though it may show a change because of SWR because R + jX changes with
> feed line length anytime the SWR is greater than 1:1.
> 73, Jerry, K0CQ
>
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