Antenna Advice

[Flash 105 FM]

I am currently running a samll transmitter connected to a folded dipole that can handle up to 100W. If I upgraded my TX to 100W, would it be advisable to also invest in a better dipole or would the one I have be capable of handling the power?

[Mosfet]

Who type or make of Dipole is it???????????????????????????????
I have many spec sheets!!!;)

[Flash 105 FM]

The thing is I didn't actually buy the antenna, it was given to me. I could have sworn it was a veronica but after checking out their website, I see that they are not the same folded dipole. This was purchased about 3/4 years ago so maybe Veronica have only recently changed their dipoles? Help would be greatly appreciated!

[Justin Speck]

If the dipole can handle 100w then presumably you can run a 100w tx into it. Don't forget you'll lose some power in your feeder so 100w won't actually reach the antenna.

Mind you, it depends how you measure power but that's probably worthy of another thread all to itself.

J

[sub-site]

Personally, I think that a serious attempt at power upgrade should always be backed up with an improved aerial type aswell as more height. Then the results will justify the effort. An increase in power alone from 25/ 30 watts to 100 watts is not worth it ie. if your range was 8 miles, your range will likely to increase to 9 miles if you are lucky and the signal/ noise ratio will be slightly better. An increase in power only from 10 watts to 100 watts will bring a clear signal to your service area than before -you will be happy with that investment. Basically, you`re gonna be disappointed if you believe more watts = more power = more range. Signal to noise ratio is determined by transmitting power.

So to obtain increased range it is probably a better idea to re-engineer your antenna. Stacked dipoles are probably the way to go and a good high aerial is the best amplification money can buy. Your load (dummy while testing) determines the ratio of *reflected* to *forward* power -has nothing to do with your amp so that could be something to be considered for keeping your current aerial or not. Also is the level of gain you currently have. 3dB is a factor of 2 increase in power, 6dB= 4X power. If your aerial is directional this is important. Although an antenna is a passive device and does not amplify, the gain which is got by concentrating the signal in a particular direction or causing the signal to be radiated at a lowever angle will result in better performance at the same power levels. (if it is any good).

Higher gain antennas give you higher "effective radiated power" (power on paper) to penetrate dwellings throught to where your listeners are, and into their antenna-less receivers. Check this out on your own antenna, look at some other specs and consider what justice your current aerial will do in this regard in comparission to other available.

Just also to note that a nine element low pass filte will handle power levels to at least 100 watts . Use this with the 30 or 40 watt amplifiers and that antennas have a certain amount of loss and are not 100% due to (I`m told) "inductance and capacitance" so not all of the power is radiated. Again, check this on your current antenna.

I hope this makes your decision a little easier.

[trev]

I have a half wave dipole with 4W FM tx and plan to upgrade soon to 3 element yagi with 7dB gain. The current range is about 5 miles. The new antenna should increase the power to at least 16W.
I only want the signal to go in one direction. Should i expect much more coverage?

[sub-site]

Your yagi should give you a 5Db of gain over a quarter wave ground-plane antenna. I don`t think you`ve subtracted your current gain which is the first thing which will diminish your results with the yagi.

It can be adjusted for a VSWR rating of 1:5:1. (4< reflected power) Maybe the match could be as good as 1:4:1. Maybe you`ll have to use a longer/ shorter feed length with the yagi. The lanscape may change against/ for your favour beyond your current range (obstruction losses/ gains), your coverage with the yagi will not be in square miles but more in distance as Yagi`s (even vertically polarised) have a forward beam that is only about 60 to 70 degrees wide.

I think the comparatively low 4 watt TX is a limiting factor on your range. On average, 4 watts is supposed to give you an ERP of about a 3 mile radius. Line of sight propagation will play it`s part too of course -there are formula available on tech. sites for calculating/ estimating this sort of thing, among others. For example: The formula for calulating power is: dB=10log(power1/ power2).

Having a fresh new yagi to replace an oldish & rusted (which some 1/2 waves are prone to) can make a difference. For a yagi, I was told to use 24 inches of sepeation if you are at the high end of the band and 20 inches if you are at the low end - Shorter seperation results in more antenna gain at the expense of lower feedpoint impedence.

[The Doc]

Sub-site, what has been your experience with the NRG 1/2 wave dipole, if any? If I increase power from 20 watts to 100 watts and increase the height of the mast by 6 or 7 feet, would there be much of a difference to coverage eg s/n ratio etc. Also you recommend the stacked di-pole array (eg SDH-100s), but what if this is not an option and I want to broadcast with a low angle of radiation omni-directionally with as much coverage area as posible from a 200ft hill (residential area) overlooking a large city.

[sub-site]

Sub-site, what has been your experience with the NRG 1/2 wave dipole, if any?
None but I hear they are prone to rusting out of tune after about a year or so, it`s also a simple aerial and doesn`t really promise much gain. (AFAIK)

If I increase power from 20 watts to 100 watts and increase the height of the mast by 6 or 7 feet, would there be much of a difference to coverage eg s/n ratio etc.
Found this article...the author explains this topic very well;

"There is no simple answer to the question "How much territory can my signal cover". The distance at which your signal can be heard will vary from listener to listener, depending on their equipment and location. In fact, many elements affect your range:
*The transmitter power,
*The transmit antenna`s height and effectiveness,
*Local terrain, (hills, large buildings etc.)
* Interference from distant stations on the same channel,
*"Splatter" from local stations on adjacent channels,
*The sensitivity of the listener`s receivers."

"On FM boradcast frequencies, antenna height puts an upper limit on your range, regardless of power levels (it is true to say that diffraction can extent your range slightly, and signals sometimes travel greater distance when atmospheric conditions are just, but we will ignore those factors for the time being.) The distance from your antenna to the radio horizon is determined by this formula; distance in miles =1.415 times the square root of the antnna height in feet."

"You can add the transmitter antenna`s range to the receive antenna`s range to predict whether reception will be possible. Example: A transmit antenna is 25 feet high, if there are no major obstructions between antenn`s and if power is high enough (or receiver sensitivity is high enough) then recetion at distance of 14 miles could be achieved."

"Now you see why Fm stations go to the expense of building antenna towers that are hundreds of feet high, or locate the antenna`s on moutain-sides that overlook the cities they want to serve."

"Now that you know the upper limit on your range resulting from your antenna height, can you calculate how strong the signal strength will be at a certain distance from the antenna? Not very easily. The following formula predicts the strength of a signal in free space at a given distance from the transmit antenna and the receive antenna. An "isotrophic" antenna is one that radiates equal amounts of power in all directions; in other words, it has a spherical radiation pattern, kind of like the sun radiating light into free space in all directions. The formula is:

E =((30*pt)^0.5/R
Where E =Field strength (in volts per meter),
pt =power accepted by the transmit antenna (in watts),
R =distance (in meters)."

"Let`s put the formula to work for us. The table below gives free space field strengths for various power levels at a distance 1 mile and 5 miles signal field strength is expressed in units called millivolts per meter (mV/m)

Power Field strength (mV/m)
(in watts) at 1 mile at 5 miles
100 34.0 6.8
50 24.1 4.8
10 10.8 2.2
5 7.6 1.5
1 3.4 0.68
0.5 2.4 0.48

Typical car radios and home stereos need a signal strength of about 1mV/m to give good reception with an acceptable signal to noise ratio. Cheap portable radios require a much stronger signal. Judging by the table above, a 1/2 wave tranmitter connected to an isotrophic antenna would provide a range of more than 1 mile for typical receivers. We would have to increase power to 10 watts if we wanted to have a similar field strength at a distance of 5 miles."

Fortunately, antennas designed for broadcasting use the transmitter power more efficiently than an isotrophic antenna. An isotrophic radiator would send equal amounts of power up into the sky, down into the ground and out towards the horizon. As a result, you can get better results than the formula predicts despending on the quality of your antenna system."

"If your antenna is located in an area where power live, improving your antenna system is always a better idea than increasing your power. Using a high power transmitter in a residential neighboerhood is likely to interfere with radio and TV receivers in the area even if your signal is technically "clean" (minimal spurs and harmonics) it can overwhelm receivers in the vicinity. (through front-end overload). If you interfere with people`s reception, some of them will complain to the authorities."

Also you recommend the stacked di-pole array (eg SDH-100s), but what if this is not an option and I want to broadcast with a low angle of radiation omni-directionally with as much coverage area as posible from a 200ft hill (residential area) overlooking a large city.
NRG also do the high-gain half-vertical antenna which will multiply your ERP by a factor of approximately 2. NRG has details on the website in it`s workshop on "monopole radiation pattern".

[3cx1500A7]

sub-site,could you supply us with the name of the publication or website from which you copy you info from please?

[sub-site]

alt.radio.pirate. Why? (never seen anyone else on this board post such a question)