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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