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Power gain and mast height gain....

TheBlaster

Well-Known Member
Jun 29, 2020
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Trying to work out some figures...

ERP is RF power + antenna gain I think...

If an antenna receives 5 Watts of RF energy with 3dB of antenna gain we get 10w.

How does height gain work ? Say an average antenna is at at 10m from ground goes to 20m on a pole I understand this gives 6dB gain. So at 160m on massive broadcast tower is that 6+6+6+6dB ?

Where does height gain start from say your antenna is 10cms from ground it is 6dB Gain at 20cms ? Something does not seem right in my reasoning.

Thanks
 

Height gain is not a real gain figure. The antenna does not produce any more gain at a higher height it simply has a less attenuated path to the receiver hence it APPEARS to have more gain at a higher height. No idea where you came up with going from 10m to 20m gives you 6dB gain. Your question about 160m and 6+6+6+6dB simply leaves me scratching my head about what you mean. In any event it does NOT do what I think it is you are assuming it does.
 
Receive gain is more of "capture ratio" more for a rejection of signals arriving to the antenna from various angles of intercept. (A figure of Merit mostly for FM but AM can also apply here for the effect is similar)

The antenna - you should focus more on the electrical properties of the fractional wavelength it is, than to think you can gain much in reception...based upon the antennas' gain figure - for you cannot predict where or how the signals arriving thru the air to your station will be picked up by the antenna.

For in light of the antenna, it's radiation pattern, and the ERP (Effective Radiated Power) means it's lobes of strongest radiation effects (the take off angles) ALSO are the antennas main lobes of reception - yes, it is reciprocal - but remember...

If You use an Omni-directional antenna - signals can be received from any angle (Perpendicular to it's axis meaning Vertical Axis antenna - Horizontal lobed reception mostly) - even if it's not using the axis of the main lobes it radiates from (This is a noise figure too).

So if you're trying to hear A STATION from one direction - and it's propagation casts a SHADOW around your antenna due to some type of refraction or reflection (Skip Zone Water Towers etc...) - you may not hear it as well as another STATION being further away, but it's propagation is not casting a shadow on your antenna from the direction it's arriving (from another direction or refraction or reflection of) (Clear open air) - you hear the station that can effectively arrive to your antenna above the noise level and other signals - this is similar to what people would call Capture in using a set of directional beams or arranged (coordinated) antennas designed to act as an array.

So think of "gain" as a figure of merit - not necessarily a performance factor - for there are many variables; in the earth, air and propagation of their signals' you're trying to receive - for antenna makers to quantifiably offer signal gain or to offer a guarantee of any value when it comes to reception - the biggest equalizers? Noise (QRM and QRN) - because antennas easily pick up even the noise you will receive in any and all directions.
 
Thanks for replies..I think my reasoning came when I searched "antenna height gain" in google... the first return displayed an excerpt a paragraph as follows:

"As a rough rule of thumb it is often said that doubling the height of an antenna will give a 6 dB increase in gain. Although this will depend upon the actual situation and a host of caveats, etc, studies have shown that it is generally not too far from the truth."
 
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http://www.dx-antennas.com/Height versus take off angle.htm

some pictures to look at for the height vs take off angle/

Thanks that is interesting but going a bit off on a tangent from original topic....

Is that correct for F2 layer max hop ? I read 1,400 miles for E layer.. 1,550 miles (2,500kms) seem a bit short in my mind given the F2 layer is 200 + miles up and E layer is...mean/average at 71 miles. That does not add up to me.

We cannot take 1,550 miles off the F2 as gospel. I see some pages reporting F2 can be 220-800kms high so it has quite a range of figures for it.
 
I often make the analogy to streetlights.

You can't always see streetlights at night from the ground where you stand, you see the "glow" from them around you.

But if you rose up in height from your location, straight up - it becomes easier to localize and identify the streetlights. You're above the obstacles and their impediments - making it easier to see them even though there are many on a busy city streets. You see the ones closest to you as the brighter ones which outshine others. But some still are blocked, that although close, have to shine thru trees or around taller buildings...casting their glow to be seen more as a cast shadow than a direct illumination.

Since light is a form of Electromagnetic Radiation (Quantum theory aside) Emission - the association you identify where the antenna as being located - is correct.

Thanks for replies..I think my reasoning came when I searched "antenna height gain" in google... the first return displayed an excerpt a paragraph as follows:

"As a rough rule of thumb it is often said that doubling the height of an antenna will give a 6 dB increase in gain. Although this will depend upon the actual situation and a host of caveats, etc, studies have shown that it is generally not too far from the truth."
 
Just letting you know from the start, this will get a bit wordy.

Trying to work out some figures...

ERP is RF power + antenna gain I think...

You got two out of three, the third thing that you need to consider is loss. This can come in many forms, in the feed line or matching system before the signal gets to the antenna, losses in the antenna while transmitting, and losses as some of your power is absorbed by the environment around the antenna. A typical mobile antenna, for example, has around 20% to 25% efficiency give or take just because of how close it is to the earth below. That is 75% to 80% of the transmitted power that is lost, and yet we can still communicate. Although in most cases, most forms of loss are applied to antenna gain.

If an antenna receives 5 Watts of RF energy with 3dB of antenna gain we get 10w.

Not exactly, although it may appear to be the case at first glance.

The first thing you have to realize is the antenna does not amplify the signal it receives. You may get 3 dB of gain in some directions, but in others you will have an equivalent loss in gain. A vertical antenna, for example, will typically take power that is transmitted straight up, a useless direction for us, and instead send more of it out towards the horizon, which is beneficial to us. Your not actually getting 10 watts out, the antenna is just directing more of the original 5 watts to a beneficial direction for you. A beam will do this even more so, cutting out much of what is radiated from the sides and back and focus it in one direction. A higher gain for an antenna is just taking more of the power going in other directions and concentrating it in what is hopefully a beneficial direction for us.

How does height gain work ? Say an average antenna is at at 10m from ground goes to 20m on a pole I understand this gives 6dB gain. So at 160m on massive broadcast tower is that 6+6+6+6dB ?

Where does height gain start from say your antenna is 10cms from ground it is 6dB Gain at 20cms ? Something does not seem right in my reasoning.

Does doubling the antennas height actually increase gain by 6 dB? For the most part no. That being said, for local communications at some heights it will seem to, but the trend doesn't hold for long. Even then getting the antenna to a wavelength in height (11 meters or about 36 feet for CB) the rate of gain (and received signal strength for local coms) really does drops off. Its not that it doesn't continue, but how fast it increases really does drop off. If you get to two wavelengths in height you are almost at the maximum gain the antenna is capable of. If you actually get the antenna high enough, in the range of 7 to 10 wavelengths above the earth depending on the antenna (higher than most people can even imagine getting their antenna), there are times when gain will actually drop.

I think Handy Andy explained it well with this line.

So think of "gain" as a figure of merit - not necessarily a performance factor - for there are many variables; in the earth, air and propagation of their signals' you're trying to receive

There really is a lot that goes in to antenna gain, but for the most part it depends on how it was measured. I can, for example, make a model and see a difference of x dB gain at y angle between two antennas. That will tell me quite a bit about how well these antennas will perform compared to each other for DX contacts coming in at said angle, but tell me nothing about local contacts. Optionally, I can take a field strength measurement with one of the modeled antennas, change it for the other and take another field strength measurement. This will tell me which antenna works better for local contacts, but tell me nothing about how well they will work for DX. Both measurements are called gain, but neither have anything to do with each other, and will often give what appears to be opposite (if not contradictory) results.

Seriously, don't get hung up over gain, and don't trust any numbers from a manufacturer. Even if they did get them from a legit method, that doesn't mean you will get said gain in your installation at home as a change in height, or in ground quality, or any number of other factors will cause it to be different at your location. The chances of you reproducing any legit published gain at home are negligible.


The DB
 
Thanks and all words understood. Yes I had in the back of the mind an antenna is not an amplifier (the opposite could be the case for some, lol.... an attenuator) And yes it refocuses the energy to different take offs/angle spreads/lobes which appears as improved signal and RX in those angles. I only deal with omni's, philosophically speaking. (for now anyway)

I suspect as you mention ground loss (part of the efficiency end results) and height above ground (ground reflection phase interactions either constructive or deconstructive) have large bearing on what angle/s are dominant.

I enjoy radio with omnis only because I like the feeling of drawing it all in (the good and the QRM etc), non discriminatory RX and TX (in some situations this has given the edge over high power beam users). i.e. getting heard and making contact on opposite side of the planet when beam users were pointing the wrong path direction, this is when you can get heard using an omni given your signal can likely be 3-4 S points down on a beam user.

I think I know what my station "weak area" is and can relatively easily resolve it but I just need to have the impetus to resolve the practical issues. For Sporadic E season and low F2 layer skip at this time it is not necessary.

As an off topic aside : (for 11m band) Come winter when summer E layer skip dies off somewhat (till maybe Christmas week as I hear.) being solar minimum what can we expect. Zero skip ? or can there be some low F2 layer openings from Fall/Winter 2020 onwards. Does Solar Minima mean no skip whatsoever or if you keep your eyes out on suns spots daily there might be the occasional day here and there ? (looks like we have a little sunspot appearing right now)

I love it ! Thanks again.
 
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Week around Christmas?

Might want to see if that is more of a tropospheric ducting event than F2 layer - because as seasons change, the events for DX propagation do change with the seasons as well.

upload_2020-7-22_9-28-4.png


As the air masses begin their transition from Summer to Winter - those Equinoxes have things going on too.

Temperature inversions, "Sporadic E" - lots of stuff can change the MUF of skip at any moment. There is more going on than just the "Sunspot" events that bring on the F2 ionization - there is also other Solar events like Coronal Holes where Solar wind "Escapes" between gaps in the Suns own corona to travel outward and if in the right direction - buffets Earths' Magnetosphere causing different events like Stratospheric Warming, Aurora - kP Index, Magneto tail reconnection events, even STEVE's :) (Particle events) which do not need to be involved with Solar Cycle - they occur more often without but offer the chance to experience "openings" even when the sun appears quiet.
Does Solar Minima mean no skip whatsoever or if you keep your eyes out on suns spots daily there might be the occasional day here and there ? (looks like we have a little sunspot appearing right now)
 

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Have a read up:
https://en.wikipedia.org/wiki/Radio_propagation
In the low sunspot years i mostly work 6 meters/10/12/15 with Sporadic E from March to august, as i wrote in several posts here before.
There is not one year in my 43 year being Ham i could not work DX on 10 or 6 or 12 or 15 some months of the year.
You just have to be more alert, we used beacons to alert us nowadays theere areseveral sites giving accurrate data about what happens where.
http://www.dxgalaxy.com/Spots/HF
 
One side of amateur radio I am not so interested in is the high level of info on LCD displays, spectrum graphs of activity etc. (I know nothing about ham radios really as I have never owned one) I am sure if you are into using multiple bands it is very useful. For me that would take the chase away to a point where it would drain the fun out of it.

Sure, sunspot minimum is a drag for those stuck on 11m. I don't think I will ever take the licence. DX on ham bands seems a bit easy IMO with beams and the organized approach it seems a bit unexciting.

Also then unless you are minted you have a compromise antenna that is not quite right for any of the bands. A good radio is $1,000.00 or more, same again for a good antenna and again for the bells and whistles.

I can do it all on 11m with the right strategy and persistence. I like to try and do 1 band as best as I can do it, it is just about enough for me although 5-6 year waits is long.

I can wait it out for the F2 DX on 11m. No choice.

And this is not a ham bashing post, it's just ham approaches do not set me on fire and I am all for band cooperation and respecting other radio users of all kinds (local chat, Pirate DX/Ham DX). In fact respect on 11m is more important than any other. It is really important not to be an idiot on 11m, have respect, some courtesy, live and let live. Don't be a QRM / abuse / whistler / envy / or turn Intl call freq a CB channel etc.
 
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No problem, everyone does what he/she likes the best.
For me to become a ham was looking for challenges, 2 meter SSB and 70 Cm SSB and 23 CM band there and on 70 mostly homebrew.
Not easy but when the results were there you did it yourself, not bought equipment.
When i was done there i went for HF and 6 meter, and the last 10 years a home made 77 feet vertical on my little plot on the edge of the city, another challenge every other ham said it can't be done, well, i did it.

We just have more bands and frequencies at our fingers, depending on conditions we go where they are, be it 160 m or 80 or up.
Spend some time on 11 meters, but not for the last 20+ years, my new radio won't even go there just transmits on Ham bands.
My choice, and since i have been around since 12 years young building stuff and I'm almost 68 been there, done it.

I don't want to be limited to one band, just try every band and it's own properties and propagation.
But that is my choice, and I respect your choice fully.

Ham radio cost money even building your own stuff, Cars cost money, clothing, guns,food all we don't want to do without.
Most transceivers were 2nd hand, only my FT 847 with collins filters and the new FT 991a are new bought.
Even being ham it can be done cheap.
Looking at peeple buying an export radio of 400 or 500 Euro here, while you can buy a new entrance FT 450 for that or a better second hand?
Some 11 meter peeps even have better stuff as me, no problem with that either.

I can pay for it, being pensioned i took care in my working life to save up and pay in my personal pension fund as well getting paid out of the Government pensionfund i paid in for 50 years.
So bought an almost new car when i retired after 15 years of 80 hour working weeks as T.I.G welder/Pipefitter, and 35 years doing other long hour jobs driving trucks through Europe, or even being Her Majesties Customs officer.
We all are mandatory insured for healthcare so no problems there.
Even saving up every month, living a good life, all paid for by working hard and long hours my whole working life.
Being an ham is a very wide world of opportunities you fill in yourself, i do what I want to do, nobody tells me what i need to do there.

On my little plot of 100 x 25 feet I put up the antenna's that worked the world on 160 - 6 meters.
2/70 all Europe, Russia, Middle East, North Africa etc.
My FT 991a is the first radio with a screen and waterfall and spectrum scope though ;)
Do I work more stations with it? nope my old transceivers do just as well, it is your knowledge of propagation and which band to use in what time that does that.
As always i listen more as yapping on the radio and learn as much I can.

AS long you and I are both happy in what we do there is no problem.
I never looked down on 11 meter people, contrary been there done that too.
We all are in the radio hobby, whatever we do.
 
One side of amateur radio I am not so interested in is the high level of info on LCD displays, spectrum graphs of activity etc. (I know nothing about ham radios really as I have never owned one) I am sure if you are into using multiple bands it is very useful. For me, that would take the chase away to a point where it would drain the fun out of it.
My main rig is a 40-year olf Yaesu Ft-101zdMKIII. Its perfect for me and I want nothing else. I have worked just about everywhere in the world on 100w and 14g wire.
 

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