About a week ago, I got the following email. While not strictly amateur radio-related, it is about antennas, and I thought some of you antenna gurus could help this guy better than I can.
Phil writes:
First, I wanna say I’ve been playing with the idea of becoming an amateur radio operator for about 50 years (don’t ask) as I’ve been a consumer electronics servicer & hobbyist for as long. At present, I (and please don’t see me as a villian) operate a FM (very low power) radio station (since 2005) om 87.9 mhz and am wondering about something; I’ve built antennae for a long time & am strangely-intrigued by a poser here.
I use a homemade dipole (1/2 wave) that is center-fed (like most), but I have taped the coax to the lower element.My results are 2 miles of coverage. Trying a folded dipole made for 88 – 108mhz only goes out about 1/2 a mile. I am wondering why.
The folded dipole is the 1byone Outdoor Radio Antenna, High Gain Omnidirectional FM Reception Antenna with Round Dipole Design. Being a commercially-built antenna I don’t understand why it gives such poor performance. I talked to the manufacturer, but they have no answers.
Also, can you tell me why my homebrew antenna works so well when it’s supposed to be a no-no to run the coax down that way? I did it for convenience. SWR is less than 1.5 and sound quality in the car to limit is very good.
I replied:
I don’t see you as a villain—as long as you’re not interfering with any licensed stations and you’re filling a community need.
Now, on to your question. I’m not an antenna guru, but I think the reason that the commercial antenna works poorly because dipoles are not meant to operated in that configuration. They are designed to straight, not curved like that. I rather doubt that the manufacturer did any real engineering or ran any kind of performance tests on this antenna, nor did they compare the results to other antennas, such as a simple dipole.
I’m also not 100% sure why your dipole configuration works with the way you run the coax down one of the elements. Perhaps the coax has really good shielding, thereby minimizing any kind of interaction. Perhaps it’s the low power level.
What I’d do if I were you is to go with a J-pole or collinear antenna. See https://www.kb6nu.com/can-help-reader-improve-2m-base-antenna/. I think most FM stations are vertically polarized anyway, aren’t they? (I’m not really sure about this.) Either antenna is going to give you some gain over a dipole and should extend your range.
If experimenting is really all you’re doing, then you really should get an amateur radio license. Experimenting is, after all, one of the reasons that ham radio exists, and you don’t run the risk (admittedly small) of running afoul of the FCC. It’s not rocket science. Just get my free study guide from kb6nu.com/study-guides.
Any of you have some other thoughts for Phil?
Alan KM4ZWW says
It looks like Phil’s “dipole” is really a off-center fed monopole, with most of the radiation coming off the coax shield. The commercial folded dipole, in the “halo” configuration is probably working OK, but is horizontally polarized and will have almost total loss to a vertical polarized receiving antenna. I agree with you a collinear array of vertical dipoles or the J-pole. If he is ambitious, he can make an array from coax:
http://www.repeater-builder.com/antenna/wa6svt.html
Walter Underwood says
Many FM broadcast stations use circular polarization. Home antennas are horizontal, car antennas are vertical, so they need to work for both.
The 1byone antenna is a halo design, using a folded dipole. The folded dipole probably raises the impedance to be a better match for the 75 Ω coax. It is horizontally polarized.
Maybe the broadcast antennas are vertically polarized. That would make the halo antenna 3-20 dB down from a vertical antenna.
The vertical dipole with the coax taped to it is probably using the coax shield as the other element, similar to a coaxial dipole. Those work fine.
It is easy to build a Yagi for FM broadcast, especially for one part of the band. I built one to monitor our college radio station. I was a DJ and engineer and my roommate was the chief engineer. I attached a folded dipole to the ceiling of our apartment with push pins, then cut wires for a director and reflector and mounted them the same way. It worked great.
Gareth Howell says
I have no specific recommendations, however, remember that there is no such thing as a free lunch. If you use a design that has “gain”, it gets that gain by focusing energy in one area at the expense of another.
A dipole is a good compromise if you want omni -directional coverage horizontally. If the listeners are in the same plane (i.e. all in two-storey houses and cars, rather than 20-storey tower-blocks), then you could introduce an aerial with gain in the vertical plane, like a J-Pole or a 5/8ths, but you’d need to be sure where the resulting lobes are pointing.
73, Gareth – M5KVK
PS Here in the UK, FM transmitters tend to be horizontally polarised as the perceived main audience was at home when the FM network was established,
Pete Babacheck says
Reading Dan’s post, sounds like (and Alan and Walter said) the homemade is a vertical and the commercial is on the flat side.
One thing that jumped out on me is the black box on the commercial. Reading the Amazon link, this is marketed as a receive antenna. Should that black box be fed?
Bill Dornbush says
I believe that the reason his antenna works well is because it is a J-Pole. See https://en.wikipedia.org/wiki/J-pole_antenna
Steve W8SFC says
I’m far from an engineer when it comes down to antenna design but I’m not sure whether any given antenna design performs differently in receive or transmission mode in this case.
Since most transceiver sets use one antenna for both functions anyway, it seems unlikely that other than wavelength considerations would make that much difference.
This circular folded antenna design is in fact being sold for FM broadcast receivers, which poses questions about the frequency ranges that it is made to perform best within, but it seems it will function as the radiating element for the original poster’s application, albeit at reduced range. This tells me it is not an efficient radiator for his use, and as several have pointed out is probably polarized by virtue of its configuration as a horizontal antenna versus vertical. Therefore it makes sense why his results are not as good with this one as were with his previous antenna set up.
He didn’t give any figures for the actual transmit power of his equipment, but I am fairly sure no antenna will get him the extended range he is seeking as much as an RF amplifier will. The question then is how much power is within FCC rules for this kind of application?
73 – Steve W8SFC
Phil Colaianni says
Hi all! I’d like to thank everybody who contributed here about the Halo vs. Dipole antenna issue. I know a little about antennae but was just puzzled as to why strapping the coax to the lower leg of that dipole would get me a 1.5-2 mile range. Now I am familiar with what’s called the vertical sleeve coax dipole & I’ve tried that with limited success.
My (so-called) configuration is a “bit” similar in that the coax, instead of running up through the lower dipole leg, is on the “outside” of it. Technically one shouldn’t think it would work very efficiently…BUT…not only does it work, but shows a 1.5 SWR!
And that ‘halo’, being directly connected to the 75ohm connector, I would think would be equally operable in transmit as well as receive simply because it’s designed for the FM band…BUT…it will not. And polarization doesn’t seem to be an issue because I mounted it on its’ side and it makes no difference regarding distance. And still it won’t “get out” more than half a mile! Huh. The SWR shows a 2 so it’s not that degratory.
But I do have one idea regarding the ‘ladder line’ suggested by KB6NU; since I can’t find a local vendor that has it where I live, I wonder if I could just some #12ga. I have and mount it to a wooden stick (dowel?) with the element spacing equal to what ladder line would have? Should work eh? Eh, I’ll give it a go when I can find the time.
But at this time I’d like to thank KB6NU and all who have responded here and offer a “spock-ism”; “live long & prosper”! 73’s to all!
-Phil-
Phil Colaianni says
Hey Steve (W8SFC)! I meant to address your closing paragraph regarding my [power] level; it is seven watts (look up CZE-7C transmitter). Being a lifelong experimenter/hobbyist in the field of electronics, radio & TV, my curiosity was peaked by just the thought of “how far should I expect to reach with seven watts?”
My situation is this; from my house to the 694 freeway is just a tad over 1.5 miles, not far from 2, actually. In the direction I’m covering the most distance is something that shouldn’t be believed. Antenna presently at ground level (literally strapped to the bedpost, LOL), much dense treeage between me and the signal limit, also many houses and apartment buildings, businesses and cell towers and let’s not forget the power lines consideration! I think I should be lucky to get 3 BLOCKS! Not the 1.5-2 miles I’m getting!
BUT…I have considered what will happen when my antenna’s elevated on a 15′ mast.
I know; flirting just a tiny bit with ol’ Fox Charlie Charlie.
But #1, I’m staying just a bit ‘below’ 88.1, #2, I broadcast nothing ‘offensive’, #3, I make sure not to cause any interference to commercial stations, and #4, my ‘transmissions’ are usually only from 9-5 my time (CDT). And even that’s not consistent.
My main aim is ‘fun’ and trying to discover why my “conditions” allow me the range I do get with my ‘pea-watter’. Because if you could see my environment, you’d probably be brave enough to place a good wager that I wouldn’t be heard a thousand FEET, let alone almost 2 miles away! But…a bumblebee is said to be too aerodynamically deficient to fly; but…the the little buggers DO it! LOL
Hey, 3’s to ya from an old CB’er! KBNR2197 -The Phillymon-
Jerry Stuckle says
Just beware that 7 watts is far beyond the limit of unlicensed (Part 15) transmitters in the FM band – and the FCC is clamping down on illegal FM transmitters with fines in the thousands of dollars, even if they don’t interfere with existing stations.