- November 19, 2017 at 7:36 pm #28450
My name is Craig, I am a new member living in Hudson NH. I’m starting my first ever entry into HF, and as such I have a couple questions – albeit somewhat academic, they might actually have purpose in the real world. 🙂
The first is regarding impedance of a dipole. I purchased an MFJ dipole for 10-meters, (the mfj-1770C) I wanted to start simple, so I bought off the shelf.
Now, of course I need to install it.
I am leaning towards installing two masts on the opposite gable ends of my home, and stringing the dipole between insulated halyards.
While reading the ARRL Handbook, one of the early discussions is surrounding impedance and the height above Earth ground. The chapter goes so far to even speak to the type of soil. (I assume wet & stony soil would absorb more RF than say a very dry sandy soil)
My real question: when the handbook says “… at only certain electrical heights above ground will the feed point impedance be 75<ohms>…”
1. Does the fact there is a wooden house between the antenna and earth ground matter… or is the house invisible?
2. If the feed point impedance starts “perfect” 75ohms, what practically happens as the electrical height of the antenna drops, and the impedance goes up? It says the resistance increases – but does that correlate to a reduction in RF radiation?
For my example of a single band 10 meter dipole, where 1/2Lambda would be roughly 16 feet, and the ridgeline of my house is also roughly 16feet AGL… would I be looking at a 32 ft mast? or, if the house is “invisible,” a 16 ft mast to just hover the dipole over the shingles will be sufficient?
Thanks very much!
Craig, N1SFTNovember 19, 2017 at 8:22 pm #28612
The best height to install a dipole is at least 1/2 wavelength above ground.
The wood in your house will be “invisible” to the RF from your antenna. However, metal that is close by and oriented in the same plane as the antenna would be something to think about. Its possible that things like electrical wiring and metal rain gutters could interact with your antenna so it would be helpful to get above the roof by 10 ft or so if you can. The other thing to consider if your antenna is right over you house would be the potential for RF to find its way onto metal objects in your home. At 10m this would like not be too big of a problem unless you use high (500w or greater) power but on lower bands (say 20m, 40m or 80m) this would be a potential problem.
On the soil, its actually the other way around. Its best to have wet, non-sandy soil as this will reflect RF waves the best and will minimize ground losses. Unfortunately, we don’t get this sort of soil conditions in most places here in New England. I hope that this helps you.
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