[OFARC] Tuning a Cobweb Antenna - need advice

[sc flowers]

Don,
Good morning and thanks for getting me to think about physics as soon as I wake up!

So, changing the surroundings of an antenna by say raising it from 7ft up to 20ft will affect the antenna system and I’d not be surprised that the SWR as well as other propagation parameters would change.

Having said that, I know that we Hams like to have 100% in our designs; however, SWR is a bit forgiving in my opinion.  By that I mean that sure we’d want 1:1 SWR if possible but 1:1.2 or even 1:1.5 are pretty darn good.  Even at 1:1.5 you’re getting 96% of your forward power going out to your antenna.

The table in this URL http://www.firestik.com/Tech_Docs/SWRLOSS.htm gives the following to add perspective:

SWR READING	% OF LOSS	ERP*	WATTS AVAILABLE
1.0:1	0.0%	100.0%	4.00
1.1:1	0.2%	99.8%	3.99
1.2:1	0.8%	99.2%	3.97
1.3:1	1.7%	98.3%	3.93
1.4:1	2.8%	97.2%	3.89
1.5:1	4.0%	96.0%	3.84

So I’d say you’ve got a pretty good SWR system as is at 20ft.  

Now as far as propagation parameters, e.g. direction, gain, elevation angle, we’d need to do antenna modeling using a program say MMANA GAL.  BTW, I am scheduled to give a presentation this March 2nd at ˜11am at BVARC ham fest (Greater Houston Ham Fest I believe it’s called).  We’ll be going over MMANA GAL antenna modeling so you may be keen.

Well, I hope that helps and again, thanks for getting me to think about physics this morning w/the first cup of coffee.

73,
Stephen (W2WF)


> On Feb 25, 2024, at 11:28 PM, Don Davis via OFARC <ofarc at mailman.qth.net> wrote:
> 
> 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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