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Low band HF multiband doublet location - height above ground vs top of hill




 
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Author Topic: Low band HF multiband doublet location - height above ground vs top of hill  (Read 383 times)
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W1EQX
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« on: January 20, 2018, 12:01:48 PM »

I have been pondering a antenna location and I would like to tap into the collective wisdom of the AM community on opinions on the placement of a multiband doublet cut for 160 meters used mainly on the lower HF bands with some operation on 20 meters and above.

I have the luxury of placing the doublet at about 75 feet above the ground between two large evergreens at the base of a sloping 80 foot hill or placing the same antenna at the top of the hill at 40 feet. Placement at the top of the hill allows open 360 degree line of sight out about 3/4 mile in the most shielded northern direction and a few miles in the most open southwest direction. The slope of hill is towards the south. If the antenna is located at the bottom of the hill it will be higher above the nearby land and open to the southwest and northeast but shielded to north and southeast.

The QTH is in the Green Mountains of Vermont. Poor, rocky soil on top and frequently water saturated prime agricultural VT loam at the bottom of the hill.

Which location would give the best overall signal results particularly on 160 through 40 meters?

Carmine
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KA2DZT
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« Reply #1 on: January 20, 2018, 12:16:42 PM »

At the bottom of the hill because it would be higher above better ground conditions.
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WD8BIL
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« Reply #2 on: January 20, 2018, 12:20:44 PM »

Can you put one up at each location?

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N1BCG
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« Reply #3 on: January 20, 2018, 12:57:54 PM »

I completely agree with DZT. Height above (away from) anything that could absorb radiated energy or affect antenna characteristics is important, particularly at lower frequencies.

Views of the horizon and elevation (HAAT) are primarily a concern for VHF and up since those signals are line of sight. For HF and down, your target is the ionosphere for skip, and those frequencies can bend around geographic obstacles to a degree.
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K1JJ
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"Let's go sailing, Tommy!" - Yaz


« Reply #4 on: January 20, 2018, 01:48:54 PM »

Carmine,

Here's what I would do:

Make up TWO dipoles, one for 160 - 75,   or 160 - 40M.       And another 20-15M, or 40 - 10M.

Put the 160-75M at 75' at the bottom of the hill.

Put the 40-10M at 40' at the top of the hill.

This will give you 20-10M at least 1/2 wave above ground, (or much more)  thus a good takeoff angle. Plus, it will be 1/2 to 1 wavelength long giving a great figure eight pattern. Otherwise, the 160M multiband antenna will give you an octopus pattern with huge nulls on the higher bands.   You will also have some tremendous low angle take offs on 20-10M due to the hill slope. Lower angles between 7 to 20 degrees are more important for 20-10 M.  This will be a great DX antenna while the 160-75M will be a reasonable/moderate local antenna. I assume this is what you are looking for.


The 160-75M dipole will be 1/8 to 1/4 (or more) wavelength above ground (for local work) and also 1/2 to 1 wave long giving a great figure 8 pattern.

It's all about compromise, but splitting the jobs up like this will possibly give you the best of both worlds.

If you are really ambitious, model it using the many Earth ground terrain programs. Also, you could always swap the antenna locations and using a reference dipole, do A/B tests in real time.

You said: "If the antenna is located at the bottom of the hill it will be higher above the nearby land and open to the southwest and northeast but shielded to north and southeast."

PERFECT!  From VT, north is very low populated and SE is over the Atlantic ocean until Asia.


Let us know what you finally come up with.

T
 
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W1EQX
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« Reply #5 on: January 28, 2018, 10:02:16 AM »

Thanks for the replies. Will give some thought.

Carmine
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