[OFARC] Tuning a Cobweb Antenna - need advice

[ralph phillips]

Don,

Most radio/antenna systems do quite well with SWR of 1.5.  Several more experienced hams have told me they don't worry about anything below 2:1

I think your tuning is good with one caveat.  I tune my 10m antenna for 28.4Mhz.  There is very little traffic above 28.5Mhz and 28.9 is where the FM enthusiasts hang out.

Ralph. KE5HDF

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________________________________
From: ofarc-bounces at mailman.qth.net <ofarc-bounces at mailman.qth.net> on behalf of Don Davis via OFARC <ofarc at mailman.qth.net>
Sent: Sunday, February 25, 2024 11:28:14 PM
To: Ofarc Reflector <ofarc at mailman.qth.net>
Subject: [OFARC] Tuning a Cobweb Antenna - need advice

Hi to All,

    Need your expertise and advice - I am in the process of tuning an MFJ 1836 cobweb antenna capable of 20M, 17M, 15M, 12M 10 M and 6M bands. I am using a telescoping mast mounted to the eave of a single story home.

 I followed the MFJ instructions for tuning the antenna about 7 feet above ground and obtained the lowest SWR possible for the center frequencies of the bands.

 However, when I installed the antenna on the mast and raised the mast about 20 feet above ground, the SWR readings for the center frequencies increased. The data is shown below.

The center frequencies are the target frequencies desired; the resonant frequencies are the lowest SWR readings.

 I have a General license, so I am tuning for the SSB frequencies in the General bands. So, the questions are:

1. Should I stand on the roof and lower the mast to be able to adjust the bands for the lowest SWR readings? Or are the readings shown below "good enough"?

2. I have read that the cobweb is "omnidirectional". But somewhere I read it should be positioned for the broad area of the antenna to face North -South or East - West, depending on the desired transmission/reception???

Thanks for your reply,

Don KI5WWD

MFJ 1836 Cobweb Antenna 20 - 6M bands
Antenna mounted 7 feet above ground for tuning using MFJ-259D antenna analyzer
General License
Band    Center Frequency                 SWR                  Rs                Resonant Frequency               SWR                  Rs
20M     14.289          1.1     48              14.284          1       50
17M     18.139          1       48              18.133          1       50
15M     21.362          1       50
12M     24.96           1       54              21.951          1       50
10M     29              1.2     42
6M      52.05           1.5     32
Antenna mounted approximately 20 feet above ground on telescoping mast.
General License
Band    Center Frequency                  SWR                 Rs                Resonant Frequency                SWR                         Rs
20M     14.289          1.5     75              14.355          1.2     55
17M     18.139          1.5     49              18.187          1.1     47
15M     21.362          1.4     39              21.381          1.2     46
12M     24.96           1.2     61              24.964          1.2     61
10M     29              1.4     61              29.023          1.2     53
6M      52.05           1.4     49              52.054          1.4     50
Band Plan for General License SSB
20M     14.255 - 14.350
17M     18.110-18.168
15M     21.275 - 21.450
12M     24.930 - 24.990
10M     28.300 - 29.700
6M      50.1 - 54.0


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