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Author Topic: An antenna idea for discussion  (Read 603 times)

WB6YCQ

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An antenna idea for discussion
« on: February 26, 2020, 11:23:01 AM »

I will be using a Yaesu FT-991A. I'm fine with the 100w, and will not be using any amplification.   I live in an HOA ( for the past 28 years ), with some interesting options:

1) I have a side slope with some decent vegetation.  I have been considering a MFJ-1795 because it would be located in an area that is relatively well concealed from sight. It is only 10' tall, and would be mounted at ground level.  My problem is few people seem to have any comments on these ( who actually have one ).  Though my 991A has a built in tuner, would I benefit from a separate auto tuner ?  Should I just try and tune it for the frequencies I might use and not have an external tuner ?

2). Another ham suggested a very different solution.  My neighbor and I have wrought iron fences on block walls along the back of our property.  There is about 70 feet of fence on each wall.  He suggested bolting a wire from each fence to a balun, and then run it through an auto antenna tuner to the rig. 

I would love input on both of these options for my HF operations.  Thanks.
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K7JQ

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Re: An antenna idea for discussion
« Reply #1 on: February 26, 2020, 01:48:24 PM »

I use a Tarheel screwdriver antenna mounted on a 70'x 6' steel fence, using it as a counterpoise...no other radials. 80-10 meters; less than 1.5:1 SWR across all motor-adjusted frequencies-no tuner needed; models that can take legal limit. Does a nice job for me at 100W. I also have one ground-mounted on a hill with 60 radials...287 countries worked with it (running a KW). I'm also HOA antenna restricted-no HOA or neighbor complaints. See my QRZ.com page for pictures.

73,  Bob K7JQ
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KH6AQ

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Re: An antenna idea for discussion
« Reply #2 on: February 26, 2020, 02:57:41 PM »

MFJ-1975
I had an MFJ-1796 which is the same antenna, plus another top hat assembly, configured as a vertical dipole. It worked well with the limitation being narrow 40 meter bandwidth.

Compared to the MFJ-1796 the MFJ-1795 could be less efficient due to ground loss. I would install 16 radials on or under the ground. The ground loss resistance may be around 10 ohms, making the minimum SWR 3:1 on 40 meters. To bring the resonant impedance up to 50 ohm a shunt inductor of roughly 1 uH can be placed across the feedpoint.

Here's the simulated 7 MHz maximum gain relative to four 10' radials placed on "medium" ground:

10' radials
4       0 dB
8    1.5 dB
16  2.8 dB
32  3.1 dB

16' radials
4    0.8 dB
8    2.2 dB
16  3.3 dB
32  3.8 dB

Using an antenna analyzer the MFJ-1795 is easy to tune. It took me one hour to tune the MFJ-1796 with just two trimming steps per band. Antenna analyzers cost only $50. The antenna can also be trimmed using the SWR meter in the FT-991A. The automatic antenna tuner in the FT-991A is all you'll need with this antenna.

Fence
Simulating your fence on 7 MHz as a 140' wire 6' above ground shows the MFJ-1795 to have more broadside gain at take off angles below 26 degrees. At the useful DX angle of 5 degrees the vertical has 7 dB greater gain. This is a little over 1 S-unit. Off the ends the dipole is at an even larger disadvantage with a 20 dB difference at a 5 degree angle. This is over 3 S-units. To tune the fence a remote autotuner such as the FT-993BRT can be used along with a 1:1 choke balun.
« Last Edit: February 26, 2020, 03:17:53 PM by KH6AQ »
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WB6YCQ

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Re: An antenna idea for discussion
« Reply #3 on: February 27, 2020, 11:20:42 AM »

I really appreciate the input.  For perspective, I was first licensed in '67 but let the license lapse in the late 80's due to CC&R restrictions.  Several months ago I restored my license & call.  I share this because in the early days I had beams, and often worked off a huge log periodic antenna, so I was a bit spoiled.

Now to the topic at hand.  The MFJ-1795, with their ground plate, should function without radials.  An alternative, though taller, would be a 4-BTV with a laydown mount, also at ground level.  It would be near a couple of palm trees, so likely wouldn't draw attention.

Two quick questions, and thanks in advance for sharing your insights:
1). Which of these, in your experiences, would perform better.  ( I can live within the confines of what I can do since it's better than nothing ).
2). Considering I'm using a Yaesu FT-991A, would I be better to invest in an antenna analyzer and just tune both for optimum performance, or getting an external auto antenna tuner ?  The 991A has an internal tuner.

Again, thanks so much for sharing.
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KH6AQ

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Re: An antenna idea for discussion
« Reply #4 on: February 28, 2020, 12:36:05 AM »

The MFJ-1795 with the MFJ-1901 base antenna mount, and a ground rod, should be good on 20, 15 and 10 meters and on 40 meters it could have around 10% radiation efficiency for an estimated ground resistance of 50 ohms.

https://www.mfjenterprises.com/Product.php?productid=MFJ-1795

https://www.mfjenterprises.com/Product.php?productid=MFJ-1901
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KH6AQ

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Re: An antenna idea for discussion
« Reply #5 on: February 28, 2020, 07:29:37 AM »

"Two quick questions, and thanks in advance for sharing your insights:
1). Which of these, in your experiences, would perform better.  ( I can live within the confines of what I can do since it's better than nothing ).

2). Considering I'm using a Yaesu FT-991A, would I be better to invest in an antenna analyzer and just tune both for optimum performance, or getting an external auto antenna tuner ?  The 991A has an internal tuner."

A1) Without radials ground loss resistance dominates the radiation efficiency and the 4BTV will perform better on 40 meters. The antennas will perform similarly on 10, 15 and 20 meters. A 7 MHz EZNEC model of each antenna connected to a 50 ohm ground (my guestimate of one ground rod) shows the 4BTV having 3 dB more gain on 40 meters. That's like doubling the transmitter power and is 1/2 of a real S-unit. 

A2) I would tune the antenna such that the FT-991A autotuner can touch up the match. A good antenna analyzer can be bought at Amazon for $50 or the antenna can be adjusted using the FT-991A SWR meter.
« Last Edit: February 28, 2020, 07:53:22 AM by KH6AQ »
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WB6YCQ

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Re: An antenna idea for discussion
« Reply #6 on: February 28, 2020, 08:03:59 AM »

Thank you.  I did order a good antenna tuner.  The MFJ-1795 is less likely to be noticed when the HOA manager cruises around looking for violations.  It is shorter, and far easier to conceal in a vegetated area. 

If I might divert from these two, the other option would be a short dipole mounted as an eastward facing inverted V in the attic.  Alpha Delta maxes a DX EE that is 40' ( each leg should then be 20' ), and RadioWavz makeds a DX40 OCF Short that is 37' ( thus about 19' per leg ).  I might even have enough space to run them both at their full lengths.  These would be totally concealed and the attic is atop a two story house so it has good elevation. 

What are your thoughts on the ground mounted vertical vs an attic mounted dipole ?  Thanks.
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KH6AQ

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Re: An antenna idea for discussion
« Reply #7 on: February 28, 2020, 04:24:08 PM »

In simulation the dipole looks pretty good. The downsides to the dipole are possibly more receive noise from home electronics and more interference to home electronics. I'd give the dipole a try. Click the link to see the 7 MHz elevation patterns (I can't get the Insert Image to work).

BLACK = MJF-1795 with one ground rod
BLUE = MFJ-1795 with 16 radials
GREEN = dipole off the end
VIOLET = dipole broadside

https://imgur.com/a/VAjW4Jp



« Last Edit: February 28, 2020, 04:33:11 PM by KH6AQ »
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WB6YCQ

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Re: An antenna idea for discussion
« Reply #8 on: February 28, 2020, 05:06:02 PM »

Thanks for all the info.  I'm going to start with the 1795 plus the ground plate for now.  I have a lot of stuff stored in the attic and it gets incredibly hot up there during the summer.  So I'm going to hit up my grandkids to free up time and clear everything out of there, and then I can clear space to properly, and safely, install the dipole. Thanks for all the info. 
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KH6AQ

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Re: An antenna idea for discussion
« Reply #9 on: February 29, 2020, 02:29:21 PM »

That sounds good and I'm interested in how you like the ground plate.
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