Post by lovinradio1985 on Mar 12, 2018 21:08:30 GMT -6
Hey guys! I need some advice on this antenna. I have been using a folded Dipole antenna and am ready to replace it. It doesn't seem to give me great coverage. It has a lot of multi path interference. It is about 46 feet up , running 25 watts. Would replacing it with the Dominator give me better range and coverage? I've heard it has 3db of gain. is this accurate? I am also mainly concerned with vehicles. I have heard that a CP type antenna is best, but have also heard that Vertical Polarization is good also. What do you folks suggest.
Post by HighMountainRadio on Mar 15, 2018 20:39:11 GMT -6
Greetings From High Mountain Radio !
After reviewing the Dominator antenna, I see that is has about a 3dbd gain figure which is a bit of an improvement over the folded dipole. According the online published information, a folded dipole antenna gain is equal to the directive gain which is 1.64 for a half wave folded dipole. So, essentially you should realize a gain of 1.36 over that of the folded dipole by using the Dominator. In order for you to increase the received signal one 's-unit' higher on a receiver signal strength meter you must increase your signal by 3 db which means you would have to double your transmit RF power ! If you consider that slight marginal increase gained with the Dominator will cost you $329.00, personally I would not consider that outlay of money worth the marginal benefit. Don't misunderstand, the Dominator seems to be a very good antenna for what it is and likely, well constructed, although I have never personally saw one in the 'flesh'. If this were an entirely new installation, it likely would be a good, solid choice and worth the investment. But since the potential benefit realized is marginal, replacing your folded dipole with it, isn't worth that much of a cash outlay but that's just my personal opinion so take it for what it's worth and the good spirit with which it is intended.
The Dominator antenna is vertically polarized, as is your folded dipole if you have it mounted in the vertical plane. Essentially you will still favor car radios which, for the most part utilize a vertical 'whip' antenna, unlike other vehicles that utilize the 'in-glass' type of antenna that is partly in the vertical plane and part of it lies in the horizontal plane. Therefore, for the most part the Dominator will favor car radios in vehicles. Of course, using a home receiver, providing that your signal is strong enough, it should be fairly well received on most home FM receivers as well. Bear in mind, many people have a cheap FM radio receiver in their home, if they even have a 'receiver' at all, and perhaps only have an 'under the counter' type of radio, or clock radio, etc. These types of radios don't have a very good FM antenna, if one at all in many cases since as long as they can hear a fair amount of stations well enough they don't even bother to attach the plastic twinlead folded dipole that comes with most half-decent and higher end FM receivers !
As you have noted, the ideal antenna that, if setup as such, a circularly polarized antenna will radiate the same amount of RF energy (power) in both the horizontal as well as vertical planes, generally in pretty much an omnidirectional pattern, just as the Dominator radiates in the vertical plane, omnidirectionally. So, the circularly polarized, or 'CP' type of antenna is used by commercial FM broadcast stations for the reason outlined. Of course, it isn't the only type of antenna used in all facilities, but the vast majority of broadcasters use CP antennas, usually multi-bay antenna arrays consisting of vertically stacked CP antennas to produce antenna gain.
That being said, if you are using 25 watts of power and use a CP single bay system which has NO gain, basically you will be delivering 12.5 watts into the vertical plane and 12.5 watts into the horizontal plane. So, pretty much to get the same coverage results using a CP antenna you will have to pretty much double your transmit power to 50 watts. Additionally, CP antennas are a lot harder to setup than a vertical or horizontal dipole or folded dipole ! I would surely not recommend it if you aren't familiar with their installation and setup already.
Personally, I would recommend saving your money and replacing the folded dipole antenna if necessary and feed it with RG-9913 50 ohm coaxial cable and use a 4:1 impedance matching balun to correctly match your folded dipole feedpoint impedance of 300 ohms to that of your 50 ohm coaxial cable, if using it, or RG-9913 low-loss coaxial cable which is 50 ohms. If you aren't familiar with impedance matching to a folded dipole (if using 50 ohm coax), I just posted a thread which describes how to fabricate the necessary impedance matching balun very cheaply utilizing a small section of 50 ohm coaxial cable configured as per the Excel spreadsheet contained in that thread.
By the way, CP antennas are a lot harder to setup than a vertical or horizontal dipole or folded dipole ! I would surely not recommend it if you aren't familiar with them already. One more note, I use 60 watts here and a homebrew ground plane antenna which is also omnidirectional and have a pretty solid 20 mile circumference coverage area. You may wish to use a Ground Plane instead of the folded dipole since it already has an input impedance of 50 ohms and no further impedance matching is required if your feedline is also 50 ohms.
It would be very helpful to know what you are feeding the folded dipole antenna with ? Is it coaxial cable, if so, what impedance is it, 50 or 75 ohms and how long is the run between your transmitter and your antenna ? If it is 50 or 75 ohm coaxial cable how are you matching the impedance of your transmission line to that of the folded dipole ? What is your VSWR reading, how much reflected power do you have ? I understand that you are running at 25 watts of power. What is your approximate coverage area with your present setup ? What is your elevation of where the transmitter is located, not how high the antenna is mounted but the height above average terrain ? Is it flatland or hilly, mountainous terrain ? Additionally, is it a more suburban or rural region or is in in or near a city ? Knowing all of these factors will help me understand the situation much better and I can advise you far better on the best course of action to take.
From my observations, many FM radio operators of this community, mostly beginners many times simply throw up an antenna of some type, whatever the cheapest coaxial cable they can find, they utilize, and fail to properly adjust the antenna for lowest possible VSWR resulting in less than stellar performance which they usually fault their 'cheap Chinese' transmitter for the so-so performance. If you take the time to properly install your broadcast antenna at the highest reasonable location and feed it with the best quality cable that you can afford and properly match it's impedance to your respective feedline, adjusting for the lowest VSWR that you can attain, usually almost anything below 2:1 will work, BUT, the lower this figure, the better !
To understand the ramifications of a mismatch between your transmitter and broadcast antenna (abnormally high VSWR) have a look here, this is an excellent online calculator !
***NOTE: To obtain an accurate measurement, VSWR should be checked at the antenna, not at the transmitter !
Here, I broadcast with 60 watts of power, currently into a homebrew Ground Plane antenna, being fed with 45 feet of Andrew FSJ2-50 3/8" Superflex Heliax with a 1.202 db per 100 feet loss figure. I use a homebrew 'balun' at the antenna feedpoint to choke and keep any RF from being radiated along the outer conductor of the feedline and I have basically a VERY 'flat' VSWR ! This means that probably 98% of that 60 watts is going out of the antenna, where you want it to go ! I have included a picture of it for your observation if you are not already familiar with one as well as a picture of my homebrew ground plane antenna when it was living in my attic, and lastly, my newly purchased 3 Element Folded Dipole Yagi antenna, which I will be mounting vertically. This one can be purchased for $14.95 on Ebay and is quite decently constructed considering the inexpensive price ! I am quite impressed to be honest, I really didn't expect it5 to be as good as it is ! I have modified it to be mounted vertically, removed the included 75-300 ohm matching transformer and made a 'coaxial balun' (Impedance Matching Transformer) so that I am able to transmit with this antenna as it was actually sold as a FM radio 'Receive Only' antenna, but with the proper and easy modifications, it lends itself very nicely to a small, low power FM station. It will very easily handle up to 200 watts !
If I can answer any questions or be of any further assistance, don't hesitate to ask ! I'd be happy to try and assist you in maximizing your coverage for the cheapest investment. I'd really enjoy getting your reply !
73... Spooky... "High Mountain Radio, Broadcasting From Somewhere High In The Remote Appalachian Mountains"
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