HFU HF Underground

Technical Topics => Equipment => Topic started by: Capt. Kidd on January 24, 2018, 0235 UTC

Title: The best undercover fm antenna on the market
Post by: Capt. Kidd 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
Title: Re: The best undercover fm antenna on the market
Post by: Pigmeat 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.

Title: Re: The best undercover fm antenna on the market
Post by: KaySeeks 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.
Title: Re: The best undercover fm antenna on the market
Post by: redhat 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.

+-RH
Title: Re: The best undercover fm antenna on the market
Post by: John Poet 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....

Title: Re: The best undercover fm antenna on the market
Post by: redhat 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
Title: Re: The best undercover fm antenna on the market
Post by: TheRelayStation 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 ?
Title: Re: The best undercover fm antenna on the market
Post by: redhat 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
Title: Re: The best undercover fm antenna on the market
Post by: Pigmeat 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?
Title: Re: The best undercover fm antenna on the market
Post by: redhat 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
Title: Re: The best undercover fm antenna on the market
Post by: Pigmeat 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.
Title: Re: The best undercover fm antenna on the market
Post by: Capt. Kidd 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
Title: Re: The best undercover fm antenna on the market
Post by: ThaDood 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.
Title: Re: The best undercover fm antenna on the market
Post by: Σ 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?
Title: Re: The best undercover fm antenna on the market
Post by: Pigmeat on January 27, 2018, 1910 UTC
You had any luck with TV line or copper tubing J-poles, Dood?
Title: Re: The best undercover fm antenna on the market
Post by: redhat on January 27, 2018, 2011 UTC
My first real FM antenna was a jpole.  I always had trouble with the match.  It turns out, how most people mount it was a part of the antenna that was not at electrical zero, and connecting that to more metal would deteriorate the match.  I later built a two bay vertical colinear anntenna out of aluminum wire, wood and plexiglass and that worked pretty well.

+-RH
Title: Re: The best undercover fm antenna on the market
Post by: Pigmeat on January 27, 2018, 2057 UTC
I've got about 500 feet of aluminum wire in the shack I use as a makeshift receiving ground. The problem is it's insulated with white teflon. Talk about standing out!

I once made a receiving dipole w/ it. The neighborhood radio Nazi's where I lived then were on me inside of week. I was supposedly "interfering" with phones and TV's. The thing had never been hooked to a transmitter, nor was it built for it.

The best was using medium sized nylon bags full of sand as longwire counterweights when we first moved here. There were rumors we were into Voodoo and the bags were talismans to keep "evil spirits" off the property.
Title: Re: The best undercover fm antenna on the market
Post by: KaySeeks on January 27, 2018, 2205 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. 

You come back at me armed with your facts and data! Darn you!  :D  All I can say is that my most recent knowledge on that subject came from a US retired broadcast guy who last worked in the 70s. That explains that.

By the way: https://en.wikipedia.org/wiki/Circular_polarization#FM_radio (https://en.wikipedia.org/wiki/Circular_polarization#FM_radio)

"The term "circular polarization" is often used erroneously to describe mixed polarity signals[citation needed] used mostly in FM radio (87.5 to 108.0 MHz in the USA), where a vertical and a horizontal component are propagated simultaneously by a single or a combined array. This has the effect of producing greater penetration into buildings and difficult reception areas than a signal with just one plane of polarization. This would be an instance where the polarization would more appropriately be called random polarization because the polarization at a receiver, although constant, will vary depending on the direction from the transmitter and other factors in the transmitting antenna design."

This reads like a optical physicist nerd getting his nickers in a twist about terminology. By that measure, most radio signals are "randomly" polarized.
Title: Re: The best undercover fm antenna on the market
Post by: redhat on January 27, 2018, 2219 UTC
As it states, this is dependent on antenna design.  ERI Rototiller type antennas are true circularly polarized antennas, same with cavity backed crossed dipole antennas.  They provide true pattern circularity.  Many of the early designs were mixed polarization, as such pattern circularity was not very uniform.  To be fair, a tower will ofter disturb the pattern uniformity by reflecting one polarization and not the other.

At the end of the day, the effect is the same.

+-RH
Title: Re: The best undercover fm antenna on the market
Post by: Pigmeat on January 27, 2018, 2244 UTC
That happens when you get engineers involved in anything, Kay. I used to teach people how to make beer. I hated it when an engineer would take my class. They weren't much trouble in the class when they were learning the basics, but by God it was twenty calls a day when they started making it on their own! They tended to rely more on the scientific method than what the finished product tasted like.

The scientific method is fine in a sanitized lab or a commercial brewhouse with a lab, but not as much in your garage or basement where all sorts of little microbial beasties are floating the air.  Malts, water, hops, yeast, and you're tastebuds are the key to good beer. Hops and malt are plants, they're different every crop, yeast mutates to adapt to it's environment during each fermentation due to any number of factors, and different little microscopic critters are floating through the air at different times of the year. Water you can get a firm handle on with treatment, but unless you're a commercial brewer, that can buy supplies in bulk and filter the air coming into your brewing and fermentation spaces, the rest is a crapshoot. Doing it the same way you got good results the last time with the same or similar ingredients is all you need to do. You can't sweat what you can't control.

It was fun to watch 'em argue over how many grams of what hops to put in for flavoring. Talk about full scale geek wars!
Title: Re: The best undercover fm antenna on the market
Post by: redhat on January 27, 2018, 2342 UTC
Good, I know who to contact once I get the hops on the fence growing!  Booze and pirate radio, what could go wrong?  8)

+-RH
Title: Re: The best undercover fm antenna on the market. DIY 1/4-wave!
Post by: ThaDood on January 29, 2018, 0434 UTC
          That dude that wanted to build 1/4-wave GND Plane planes. Ya really wanna' see how I do it? (For RX'ing purposes, right? Hi-hi...) Here's one (Of many.), site(s) to show how to build a 1/4-wave Ground Plane.      http://www.localmeridian.com/2016/06/build-your-own-antenna-14-wave-vhf-ground-plane-antenna/          I, however, find my GND Radials and Vertical Radiator lengths a bit differently using speed of light (In Meters per second, 300,000,000.), over frequency (In MHz.). The four GND Radials are calculated 1st. BTW, they can be any metallic material. Copper and brass are solderable, while I've seen steel welded. Let's use the FREQ 98.1MHz, for example. Let's drop 6 decimal places and do 300 / 98.100 =  3.058 Meters for full wave length. We want 1/4-wave, so divide by 4 =  .765M, or 76.5cm. For standard size, .765 X 39.4 = 30.122, or 30 1/8". So, each GND Radial is going to be 30 1/8 inches in length. (As seen in that preceding website, fashion into 45 degree. Now, for the Vertical Radiator, I use #10 AWG solid copper bare wire. Why??? Cause, I've had very good luck with it, and I can't afford to make it out of the next best conductor, silver. We know that each 45 degree GND Radials will be 30 1/8" for the FREQ of 98.1MHz. If we were in outer space (A vacuum.), then we could make the Vertical Radiator that length, but to really fine-tweak out VSWR match to 1:1.1 (As possible.), I compensate for being at sea level with an atmospheric pressure of 14.7psi. How? Deduct  -5% of length. So, 30.122 - 5% = 28.616, or 28  5/8 inches (Close enough.). And, what coax can be used? Mini 8 (RG-8X), works well since it's less lossy than the 2M HAM 144MHz FREQ's and especially VHF-High TV FREQ's. And, it's affordable. I do suggest using using teflon PL-259 and SO-239 connectors. Do no weather proofing directly on RF solder points with silicon caulk, or tapes, as they effect the RF skin conductivity. Heat shrinks and cured Liquid Tape actually works well here.  Get it? Got it? GOOD!!! Now, start building!
Title: Re: The best undercover fm antenna on the market
Post by: Pigmeat on January 29, 2018, 0513 UTC
Remind me to send you all my wire coat hangers, Dood. We'll put together an array for 11 meters that will reach beyond Pluto on 4 watts.
Title: Re: The best undercover fm antenna on the market
Post by: Josh on January 31, 2018, 1720 UTC
Wouldn't a halfwave vert or 5/8ths be better for coverage than a 1/4?
Title: Re: The best undercover fm antenna on the market
Post by: redhat on January 31, 2018, 2301 UTC
Back in the day I had a lot of trouble building FM antennas that would resonate anywhere close to the design frequency.  The trouble I later discovered was something called K factor.  The larger the diameter the radiating element, the shorter the element needed for resonance.  There is a chart in one of the ARRL handbooks that has a table.  I seem to recall with 3/4" pipe at FM frequencies, around 93% of the free space length is required.

This applies to HF too, but usually unless you are using a large face tower, the effect is pretty small.

I never liked groundplane or 5/8 wave antennas at FM, I've always found their radiation patters to be too elevated to be useful.  Again, a lot of your hard earned power is going up around 30-45 degrees above horizon, not real useful.  They also radiate some secondary lobes that can cause multipath problems.

Dipoles are less trouble anyway.

+-RH