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Author Topic: The best undercover fm antenna on the market  (Read 2364 times)

Offline redhat

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Re: The best undercover fm antenna on the market
« Reply #15 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
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WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
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 #16 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.

Offline KaySeeks

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Re: The best undercover fm antenna on the market
« Reply #17 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

"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.
Just somebody with a radio, a computer and a pair of headphones...

Offline redhat

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Re: The best undercover fm antenna on the market
« Reply #18 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
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
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 #19 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!

Offline redhat

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Re: The best undercover fm antenna on the market
« Reply #20 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
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Offline ThaDood

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Re: The best undercover fm antenna on the market. DIY 1/4-wave!
« Reply #21 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!
I can't decide upon what's worst, young and stupid, or old and chemically dumbed down.

Offline Pigmeat

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Re: The best undercover fm antenna on the market
« Reply #22 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.

Offline Josh

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Re: The best undercover fm antenna on the market
« Reply #23 on: January 31, 2018, 1720 UTC »
Wouldn't a halfwave vert or 5/8ths be better for coverage than a 1/4?
Conveniently located near Vincennes Indiana.

Offline redhat

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Re: The best undercover fm antenna on the market
« Reply #24 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
« Last Edit: January 31, 2018, 2308 UTC by redhat »
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com