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Author Topic: The best undercover fm antenna on the market  (Read 2369 times)

Offline Capt. Kidd

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The best undercover fm antenna on the market
« on: January 24, 2018, 0235 UTC »
So a little while ago I saw this little dipole on eBay for only $99 and I thought Id pick one up.
Heres a link to the same thing on amazon for a bit more but you can occasionally find these floating around eBay for much less
https://www.amazon.com/gp/aw/d/B00NNERREM/ref=mp_s_a_1_1?ie=UTF8&qid=1516760244&sr=8-1&pi=AC_SX236_SY340_QL65&keywords=fm+dipole+antenna+150+watts&dpPl=1&dpID=21UawYVmTiL&ref=plSrch
When t bought this I connected it to an old 15 watt fm tx I had just to see what would happen. I thought I wouldnt see much of on improvement over the little telescope antenna that came with the thing. I WAS WORNG! The range jumped from about 1.5 miles to at least 8. While trying to find the range I actually hade to turn around because my truck was about to run out of gas and I was in the middle of nowhere and at that point the signal was coming through perfectly. Id also like to point out that during the test the antenna was attached to a bench press in my attic far away from the spying eyes of the fcc. So to sum up. 15 watts of power a relatively cheap (possibly used) antenna a TNC to pl259 adapter and about 3 feet of rg8u and you get better than 8 miles. Enough to cover my little town. All added up an the whole set up cast me about $250
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Offline Pigmeat

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Re: The best undercover fm antenna on the market
« Reply #1 on: January 24, 2018, 1928 UTC »
It looks like an adjustable version of the transmitting dipole Ramsey used to sell before they shut their doors.

Vertical dipoles do well on FM. I used to screw around with an FM transmitter for kicks, you couldn't hear much beyond the yard with the whip that came with it. I built  a vertical dipole, hung it outside a second story window, fed it with a short section of coax and my range jumped from about 150 yards to a half mile from that alone. The kicker is it was all legal under Part-15 rules. Tx power, antenna and total length of feedline, all well within FCC rules. In a densely packed urban environment you could have quite a nice little audience with that alone.


Offline KaySeeks

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Re: The best undercover fm antenna on the market
« Reply #2 on: January 24, 2018, 2102 UTC »
Vertical dipoles do well on FM.

Pretty sure that most commercial FM broadcast in the US is vertically polarized anyway so conforming to that makes sense if you want to attract listeners.

Back "in the day", I think I used an antenna that was a combination of horizontally and vertically polarized (because that's what I had available) with much, much less than 1 Watt and I think the range was about 1.5-2 miles (~2.5-3 km) but the TX was on a hill. Your mileage may vary, as they say.
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Offline redhat

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Re: The best undercover fm antenna on the market
« Reply #3 on: January 25, 2018, 0751 UTC »

Pretty sure that most commercial FM broadcast in the US is vertically polarized anyway so conforming to that makes sense if you want to attract listeners.


Not true.  Most commercial FM stations use right hand circular polarization and have done so since the late 70's.  Early FM stations were almost always horizontally polarized, borrowing from the TV practice that most human made noise is vertically polarized.  When car radios became more popular, and FM became the dominant band in the 70's, broadcasters discovered that CP could make mobile listening better, as most receive antennas on cars were vertically polarized.  Since CP will excite both vertically and horizontally polarized antennas, and has the advantage of better building penetration and lower multipath distortion it its still used today.  The majority of stations that broadcast exclusively in the vertical mode in the US are those stations which were built on the low end of the band (88-92 MHz) and within a channel 6 (82-88 MHz) protection area, and utilized vertical polarization to protect a horizontally polarized television signal.

It's not all roses.  CP antennas require twice the input power than a conventionally polarized one to achieve the same level of ERP.  Early ring and stub antennas used for CP service were also very prone to icing events, as their design was pretty narrowband.

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« Last Edit: January 25, 2018, 0756 UTC by redhat »
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Offline John Poet

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Re: The best undercover fm antenna on the market
« Reply #4 on: January 25, 2018, 1859 UTC »
Nice work!

Much is dependent on the actual elevation of your site and the terrain in general, as well as antenna height.  If the site happens to sit in a low spot, one might do much less well than you are doing with the same 15 watts.... so I heard once long ago....


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Offline redhat

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Re: The best undercover fm antenna on the market
« Reply #5 on: January 25, 2018, 2121 UTC »
True.  'back in the day' I had close to 1 KW of ERP, but at 50' it didn't travel that far.  I have on occasion heard it up to 60 miles away when up on a ridge when local enhancement was active, but you had to know what you were listening for to hear it.

Early on, I helped build a LP100 class station with a vertical antenna.  In the fringe areas there was a lot of picket fencing (multipath).  When we switched to a CP antenna, most of that went away.

I also noted from my own experience that with CP I had a lot fewer holes in my coverage area.

+-RH
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Offline TheRelayStation

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Re: The best undercover fm antenna on the market
« Reply #6 on: January 25, 2018, 2137 UTC »
CP antenna was designed with moving targets in mind, that being said, operating under 1KW ERP on a vertical antenna versus CP antenna wouldnt really matter too much since the area coverage is small at that level of power, the only real world difference is with a moving target, antenna height matters more with an ERP under 1KW.
at under 1KW ERP using a CP antenna, how long could a moving target at 40MPH hear the station clearly before multipath occurs versus a vertical at under 1KW ERP ?
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Offline redhat

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Re: The best undercover fm antenna on the market
« Reply #7 on: January 25, 2018, 2237 UTC »
3-4 miles was typical, although as you got out of town the signal would recover for a while and finally fade out as you ran out of fresnel clearance.  In places of heavy multipath, the CP wavefront would reflect off nearby objects and produce a degree of null fill.  It was effective enough for me to build a two bay antenna and run for several years until the FM's closed.  I never again considered vertical only transmit antennas unless for emergency or testing applications.

+-RH
« Last Edit: January 25, 2018, 2255 UTC by redhat »
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Offline Pigmeat

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Re: The best undercover fm antenna on the market
« Reply #8 on: January 26, 2018, 0621 UTC »
Interesting. I set that dipole up as an inverted V a couple of times to check on a combined vertical and horizontal polarized antenna that was simple to play with. It did have better reception in what had been some fringe areas when out driving around, but it also had pronounced lobes of what seemed to be the general direction the legs were pointed in? It may have been the neighbor's metal roof, who knows?

Offline redhat

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Re: The best undercover fm antenna on the market
« Reply #9 on: January 26, 2018, 2205 UTC »
I could see a problem with an inverted V on 3M as like on HF a good chunk of your hard earned transmitter power is going straight up, or nearly so.  Unless you have a flying audience, perhaps not the best choice.

+-RH
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Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Offline Pigmeat

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Re: The best undercover fm antenna on the market
« Reply #10 on: January 27, 2018, 0046 UTC »
It caused some RF in the studio, too, which is why it didn't last long as an inverted V.

A chunk of it is part of an DIY HDTV antenna in the attic these days. It doesn't do bad, but I live on the end of a ridge. I could use a pair of adjustable rabbit ears and a preamp and get most of the over the air stuff here from where I'm at.

Offline Capt. Kidd

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Re: The best undercover fm antenna on the market
« Reply #11 on: January 27, 2018, 0306 UTC »
Nice work!

Much is dependent on the actual elevation of your site and the terrain in general, as well as antenna height.  If the site happens to sit in a low spot, one might do much less well than you are doing with the same 15 watts.... so I heard once long ago....
Thanks! it was in my attic so Id say about 12 feet up
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Offline ThaDood

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Re: The best undercover fm antenna on the market
« Reply #12 on: January 27, 2018, 1733 UTC »
       The best FM undercover antenna that I've ever had was a custom FM 1/4-wave GND Plane that was put way up in a pine tree. (Actual, I've built more than one for other folks as well.) Performance and match were both great, and unless you knew exactly where to look, well hidden. Cost? Less than $10.00 each, with most of my parts on hand.
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Offline Σ

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Re: The best undercover fm antenna on the market
« Reply #13 on: January 27, 2018, 1742 UTC »
       The best FM undercover antenna that I've ever had was a custom FM 1/4-wave GND Plane that was put way up in a pine tree. (Actual, I've built more than one for other folks as well.) Performance and match were both great, and unless you knew exactly where to look, well hidden. Cost? Less than $10.00 each, with most of my parts on hand.

Care to share the construction details?   :)  Also, what did you use for a feedline to keep the losses down?
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Offline Pigmeat

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Re: The best undercover fm antenna on the market
« Reply #14 on: January 27, 2018, 1910 UTC »
You had any luck with TV line or copper tubing J-poles, Dood?