Jump to content

BuyTwoWayRadios.com

Photo

Understanding SWR & How Antennas Work


  • Please log in to reply
35 replies to this topic

#1 marcspaz

marcspaz

    Advanced Member

  • Members
  • PipPipPip
  • 663 posts
  • LocationLocation Location

Posted 20 July 2020 - 11:10 PM

Hey folks... I have been seeing a lot of antenna related posts lately. Some info I have read lead me to create this post.  I want to try to help people understand design types and how antenna SWR and Gain are impacted.  I am not going to get too technical, because I don't want new people to feel lost or leave with more questions than answers.
 
As an FYI, while the concepts apply to all antenna types, I will be focusing on vertical antennas, such as what we use on our vehicles.
 
First, lets discuss basic antenna standards and why antenna length matters.  The best way to describe how basic antenna length is relevant, is by comparing an antenna to a speaker.  Pretty much everyone understands that a speaker vibrates to make noise. We also understand that small speakers do a better job at making very high frequency sounds (tweeters) and really big speakers are better for making very low frequency sounds (like a sub-woofer).  Antennas are the same way.  The lower in frequency, the bigger (or longer) the antenna.
 
The reason for this is because, like a speaker, antennas resonate (or vibrate) the best at one very specific frequency.  As you go higher or lower in frequency, you are moving away from the antennas resonant frequency  This becomes important for several reasons.  One is because the closer the antenna's resonate frequency is to the frequency you want to transmit on or receive on, the more range and fidelity you get.  Another reason is because the energy that gets sent to the antenna must go somewhere.  If the antenna is not at the correct length to vibrate at the desired frequency, that energy gets wasted by being "reflected" back into transmitters, as well as desensitizing receivers. 
 
Any energy that gets reflected back into the radio is typically identified by the Standing Wave Ratio (SWR) or the ratio of transmitted (forward) energy vs. reflected (reverse) energy.  Here is why we watch SWR.  If your transmitter is putting out 50 watts, and your meter says you have an SWR of 1.7:1, that means only 45 watts of energy leaves the antenna and 5 watts goes back into the transmitter. 
 
A high SWR not only causes power loss, but it also generates heat as well as applying reverse electrical energy to the parts.  If enough of a percentage gets reflected back into the transmitter, it breaks.  It is very well documented that with the current technology we have, the threshold is an SWR of about 3.0:1.
 
So, now that we understand, on a very basic level, why antennas need to be a specific length to work the best, and also have a basic understanding of what SWR is, lets discuss antenna design.
 
So, what's the standard?  An isotropic antenna.  This is a theoretical antenna that radiates equally in all directions with the same intensity.  Basically, a perfect sphere.  The antenna is said to have a power gain of 1 in the spherical space all around it and has an efficiency of 100%. The concept of an isotropic antenna is often used as a reference antenna for the antenna gain.
 
What is antenna gain?  Glad you asked!  There is a lot of science behind that... so I am not going to bore you with science.  Instead, lets talk about food! Everyone loves food and its pretty easy to understand.
 
So, the concept of gain is this... you only have 100% of your energy available.  There is no such thing as an antenna magically giving you more power.  You know the perfect sphere radiation pattern mentioned earlier... well in the real world, the closet we have ever come to creating that, actually looks more like a doughnut.  Imagine a perfect doughnut. 
 
Sounds yummy right?  Well, you only have 100% of the doughnut.  What do you do if you want the doughnut to be wider, say... to fill a box better from side to side?  I mean, its a whole doughnut.  Easy... you squish it from the top and bottom.  Then the doughnut gets shorter from top to bottom, but the food has to go somewhere.  So, it spreads out wider or "gains" width in sacrifice of height.  Well the more you squeeze from the top and bottom, the wider it gets, but loses height until the doughnut is perfectly flat and the 100% of the doughnut as been spread as far as possible.
 
The squished doughnut thing makes sense, right?  Antennas that have "gain" do the same thing.  They squish the radio energy doughnut, forcing it to be wider to cover more distance, but at the sacrifice of signal height.  This means that while you can transmit and receive further side to side, you lose elevation. 
 
Lets imagine you are at the bottom of a hill and your buddy is at the top.  If you don't squish the doughnut, he can hear you because the doughnut is at its full height.  But if you squish the doughnut, people further away at your level will now hear you, but your buddy who is very close at the top of the hill will not. 
 
So, gain has a trade-off.  If you live in a hilly or mountainous area, you may want to avoid high gain antennas, so as your elevation changes, you are less likely to lose touch with someone.  Compare that to being on the water, in flat(ish) desert or talking aircraft to aircraft, you may want a very high gain antenna, because there will be no significant elevation differences. 
 
Now, from here, we could talk about the benefits of stacked phase element antennas, takeoff angles and a bunch of other stuff.  However, unless you have a more advanced understanding of antenna propagation and design, and plan on getting into some high-tech stuff, it will likely cause more confusion.  Not to mention, for what we are doing... those items are almost not relevant when it comes to helping you pick the correct antenna for your application. 
 
So, let talk about how gain and SWR can really be confusing and how numbers can trick you into making a mistake. 
 
Remember when we discussed antennas needing to be a specific length to resonate at the desired frequency?  Well, many high gain designs cause the antenna to properly resonate at only small segments of the frequency spectrum. Basically, what these means is (as an example) instead of being resonant and having good SWR across 100 megahertz, the antenna design may cause the antenna to only be resonant and have proper SWR at 10 small groups of frequencies inside that same 100 megahertz range.
 
Watch the two videos linked below for a better understanding.  In the first video, I have a Diamond NR7900A mobile antenna.  It has 3.7db gain on VHF in the 140MHz-160MHz range, and 6.4db gain on UHF in the range of about 440MHz-500MHz.
 
You will see that while measuring the SWR (or the antenna and cable resonance) you will see that in the VHF segment, the SWR varies somewhat quickly, but only has a single swing, from high to low and back to high inside of about 20MHz.  When I switch to UHF and test the higher gain portion of the antenna, you will see the the SWR bounce up and down a few times as I sweep about 30MHz. 
 
This shows that the higher the gain, the more the antenna design may actually make it so the antenna is not usable on your desired frequency.  Of course this all varies by brand and model, but the principle is still universally true.
 
https://www.youtube....h?v=Rh6w46VM_Ng
 
Now, in this second video, we are looking at a UHF 1/4 wave antenna.  This antenna is considered to have a gain factor of 1, and any number times 1 equals itself... so effectively, no gain.  You can see that we sweep over 50MHz and while the SWR wavers a little, the SWR is stable compared to the high gain antenna and 100% safely usable through the whole spectrum, never going over 1.4:1 from 440MHz to 470MHz.  Again, reinforcing the idea that the closer the antenna is to the desired resonant frequency and the less you squish the doughnut, the broader the usable frequency range will be, the broader the geographical coverage will be, the less risk of losing power due to poor SWR, and less risk of damaging your transmitter.
 
https://www.youtube....h?v=P5GiPLzVzbg
 
So, to wrap this up, I want to discuss antenna tuning.  Some antennas may need to be cut to the proper length to resonate on the desired frequencies.  This is typically done with antennas that either have no loading coils (1/4, 5/8, 1/2, 7/8 wave length antennas for example) as well as some bottom loaded antennas. 
 
A bottom loaded antenna is an antenna that has a whip that is not the correct physical length to be resonant on a desired frequency, but the coil of wire on the bottom makes the antenna electrically the correct length.  General speaking, for VHF and UHF, I recommend staying away from antennas with coils in them if they have been physically shortened for looks/clearance reasons.  These antenna work, but are not very good performers. 
 
That said, there are some antennas that are "pre-tuned" at the factory to perform correctly in the indicated frequency range.  These are typically gain antennas that have a collection of coils and capacitors on the antenna to help create the phasing and properly stack the elements.  These are referred to as LC networked antennas.
 
If you have an antenna that has stacked phasing and/or LC networks and you can't get a good SWR... unless the manufacturer provides directions on how to properly do so, do NOT trim the antenna to try to achieve the proper resonance, as you will only damage the antenna.  The coils, capacitors and whip elements are precisely cut to work together.  If you do not get a good SWR, either you need to pick a new location to install the antenna, the antenna is not properly grounded or the antenna is damage and should be replaced.
 
If you do trim a stacked phasing and/or LC network antenna, in most occasions, the antenna never gets to the proper length regardless of how much you cut the whip and you end up tossing the antenna in the trash.  If you get it tuned to a target frequency, it's usable bandwidth will be so small that the antenna will not have any real value.  I have seen some people try to tune high-gain antennas, get them tuned to about 1.7:1 or even 1.9:1 and as soon as they tune 10KHz in one direction or another the SWR skyrockets. 
 
I hope this helps with some of the antenna questions.


  • PRadio, berkinet, Elkhunter521 and 9 others like this

#2 tweiss3

tweiss3

    Advanced Member

  • Members
  • PipPipPip
  • 126 posts
  • LocationOhio
  • GMRS Callsign:WRFV692
  • Ham Callsign:KE8OWX

Posted 21 July 2020 - 07:10 AM

While I understand this, starting out I did not. 

 

I appreciate your description. It should make it easy for anyone to understand. People must understand, you cannot make energy appear or disappear. It can be lost by converting it to heat (via line loss or SWR). While many want to "squeze the most" of out their radio by getting high gain antennas, they must first evaluate their operating conditions. Cases where no gain (1db) would be ideal may be mixed use (both simplex to another radio and repeater usage) as well as high elevation changes where you drive or ride (such as here in Ohio, most of PA, mountainous areas). Places you might get away with high gain antennas include the great plains, FL, places you can see straight for miles and miles. 

 

I would like to add, for the above reasons, if you don't want to think about it and have great consistent all over use while driving, wheeling, riding, etc, perhaps stick with no gain antennas. Remember, even if you can get 30 miles simplex, the other station must also be able to respond to you for communication to work. Even if you can squeeze that extra distance, the other party may have a no gain antenna and may not get back to you.

 

Keep in mind, for GMRS 1/4 wave antennas are approximately 6" (462.00MHz = 6.39", 467.00MHz = 6.32") and provide wide bandwidth and 1db gain. 


  • Mikeam and Gerhard like this

#3 GuySagi

GuySagi

    Advanced Member

  • Members
  • PipPipPip
  • 55 posts
  • GMRS Callsign:WRFZ267

Posted 21 July 2020 - 07:46 AM

Great explanation, thank you. 


  • marcspaz likes this

#4 Mikeam

Mikeam

    Advanced Member

  • Members
  • PipPipPip
  • 56 posts
  • LocationWashington State
  • GMRS Callsign:WRFF390

Posted 21 July 2020 - 10:08 AM

Great information and well written, thanks!!!!!


  • marcspaz likes this

#5 axorlov

axorlov

    Advanced Member

  • Members
  • PipPipPip
  • 139 posts
  • LocationLivermore, CA
  • GMRS Callsign:WQVX408
  • Ham Callsign:KM6JGU

Posted 22 July 2020 - 01:41 AM

Marc, I applaud the effort, but you've got the "gain" thing incorrect. Gain is a relative measure: something over the reference. And decibel is a logarithmic unit. "my antenna has 1db gain" translated to English means "intensity of the field created by my antenna is 1.26 times higher than the intensity of the field created by <what exactly???>".

0 dB = no gain over whatever reference you are using

1 dBi = 1 decibel over isotropic antenna ("i" is for isotropic)

1 dBd = 1 decibel over half-wave dipole ("d" is for dipole)

Half-wave dipole has 2.15 dBi gain, that is 2.15 dB over isotropic antenna, and the intensity field is shaped like doughnut

Quarter-wave monopole over ideal ground has 2.15 dBi gain, and the field is shaped like the upper half of the doughnut

 

1/2-wave and 1/4-wave antennas often called "no gain" or "unity gain" because they have 0 dBd gain (who might have thought...).

 

Manufacturers and resellers may specify dBd or dBi. When they just specify dB it is better to assume that dBi is stated.


  • Jones, berkinet, Radioguy7268 and 1 other like this

- Alex


#6 NV_Rocketeer

NV_Rocketeer

    Member

  • Members
  • PipPip
  • 17 posts
  • LocationSparks, NV
  • GMRS Callsign:WRHR956

Posted 22 July 2020 - 08:18 AM

I hope this helps with some of the antenna questions.

 

Thank you, Marc! That was very lucid.



#7 Jones

Jones

    Advanced Member

  • Members
  • PipPipPip
  • 427 posts
  • LocationBetween 2 large corn fields
  • GMRS Callsign:WQYM541
  • Ham Callsign:KB0HAW

Posted 22 July 2020 - 09:16 AM

Thanks for the writeup Marc, very good.   ..except I have to agree with Axorlov, a quarter-wave antenna has a gain FACTOR of one, not a gain of 1dB. 

 

A gain factor of one, is in fact, zero dBd, or 2.15 dBi.

 

An antenna that is rated for 3 dBd gain would have a gain factor of 2, thus a 10 Watt transmitter going into a 3dBd gain antenna would produce 20 Watts ERP. (effective radiated power)  Each 3 dB of gain represents a doubling of power. Each 3 dB of loss equals a halving of power.


  • SteveC7010 likes this

#8 marcspaz

marcspaz

    Advanced Member

  • Members
  • PipPipPip
  • 663 posts
  • LocationLocation Location

Posted 22 July 2020 - 10:50 AM

You are correct an I will correct it.  Gain factor of an isotropic antenna is 1.0, not one BD... typed a lot.  type-os happen. 

 

You guys are killing me, though.  I specifically said I didn't want to science it up so I didn't confuse people.  SMH  Explain stuff like a little kid is reading it so people who have no idea what is being discussed can at least comprehend the general concept.  All that stuff you guys posted is wasted on me.  I already know it.  The target audience is not going to have a clue what you wrote means.

 

 

 

EDIT:

but you've got the "gain" thing incorrect.


That's not true... what I described about the radiation pattern and its general shape as gain increases is correct.  I buggered a type-o.  That doesn't make the rest explanation of incorrect.


  • Mikeam likes this

#9 BoxCar

BoxCar

    Advanced Member

  • Members
  • PipPipPip
  • 181 posts
  • LocationArden, NC
  • GMRS Callsign:WRCM737

Posted 22 July 2020 - 11:36 AM

Marc, I'm a technical type myself and I did pick up on the misstatements but I also wasn't going to nit-pick a great analogy apart over what, in the great scheme of things, were fairly minor points. It was, and still is, a great non-technical write-up of a very technical topic.


  • marcspaz and wayoverthere like this

Old and wise infers you were once young and stupid


#10 marcspaz

marcspaz

    Advanced Member

  • Members
  • PipPipPip
  • 663 posts
  • LocationLocation Location

Posted 22 July 2020 - 11:51 AM

Marc, I'm a technical type myself and I did pick up on the misstatements but I also wasn't going to nit-pick a great analogy apart over what, in the great scheme of things, were fairly minor points. It was, and still is, a great non-technical write-up of a very technical topic.




If I make a mistake, please let me know. I do this stuff all day as a professional and sometimes the brain gets fried. Especially at 12:00 AM.

#11 axorlov

axorlov

    Advanced Member

  • Members
  • PipPipPip
  • 139 posts
  • LocationLivermore, CA
  • GMRS Callsign:WQVX408
  • Ham Callsign:KM6JGU

Posted 22 July 2020 - 12:18 PM

"People are akin to sausages - what they are stuffed with, they keep on carrying inside" - said some dude long time ago. It is very important to be factually correct, when putting together explanatory educational post. The gain factor, the dB measure are touted around often, it is not an unimportant detail. And I did not object to explanations and analogies.

 

You are correct an I will correct it.  Gain factor of an isotropic antenna is 1.0, not one BD... typed a lot.  type-os happen. 

 

You guys are killing me, though.  I specifically said I didn't want to science it up so I didn't confuse people.  SMH  Explain stuff like a little kid is reading it so people who have no idea what is being discussed can at least comprehend the general concept.  All that stuff you guys posted is wasted on me.  I already know it.  The target audience is not going to have a clue what you wrote means.

 

 

 

EDIT:

That's not true... what I described about the radiation pattern and its general shape as gain increases is correct.  I buggered a type-o.  That doesn't make the rest explanation of incorrect.


  • WRAM373 - Richard likes this

- Alex


#12 Lscott

Lscott

    Advanced Member

  • Members
  • PipPipPip
  • 305 posts
  • GMRS Callsign:WRBZ532
  • Ham Callsign:KC8LDO

Posted 22 July 2020 - 12:42 PM

Marc, I'm a technical type myself and I did pick up on the misstatements but I also wasn't going to nit-pick a great analogy apart over what, in the great scheme of things, were fairly minor points. It was, and still is, a great non-technical write-up of a very technical topic.

I agree. For the most part it did what his intended purpose was, a very basic explanation of antenna tuning and gain. Since it wasn't meant as a highly technical article minor errors can slide by without a major loss in the concepts. Great effort.



#13 marcspaz

marcspaz

    Advanced Member

  • Members
  • PipPipPip
  • 663 posts
  • LocationLocation Location

Posted 22 July 2020 - 01:15 PM

"People are akin to sausages - what they are stuffed with, they keep on carrying inside" - said some dude long time ago. It is very important to be factually correct, when putting together explanatory educational post. The gain factor, the dB measure are touted around often, it is not an unimportant detail. And I did not object to explanations and analogies.



That is the most politely anyone has ever told me that I'm full of crap.  LOL


  • Elkhunter521 and Mikeam like this

#14 Jones

Jones

    Advanced Member

  • Members
  • PipPipPip
  • 427 posts
  • LocationBetween 2 large corn fields
  • GMRS Callsign:WQYM541
  • Ham Callsign:KB0HAW

Posted 22 July 2020 - 03:12 PM

Marc, I must add that I am a also very technical person, being a broadcast engineer, but that doughnut analogy of yours is spot on.  A flattened doughnut will spread its contents outward from the center point - just like a stacked vertical antenna array - no doubt about it.

 

Now... uh, where are those doughnuts?  In my world, 12 dB = one dozen doughnut balls. Mmmm.  I'll take those with RC Cola please.


  • marcspaz and NV_Rocketeer like this

#15 marcspaz

marcspaz

    Advanced Member

  • Members
  • PipPipPip
  • 663 posts
  • LocationLocation Location

Posted 22 July 2020 - 03:18 PM

Marc, I must add that I am a also very technical person, being a broadcast engineer, but that doughnut analogy of yours is spot on.  A flattened doughnut will spread its contents outward from the center point - just like a stacked vertical antenna array - no doubt about it.

 

Now... uh, where are those doughnuts?  In my world, 12 dB = one dozen doughnut balls. Mmmm.  I'll take those with RC Cola please.

 

I wish we all lived closer.  I would love to sit down and chat with many of you.  There is a lot of knowledge and experience in this group.  I would definitely bring some doughnuts.


  • Jones and fremont like this

#16 Jones

Jones

    Advanced Member

  • Members
  • PipPipPip
  • 427 posts
  • LocationBetween 2 large corn fields
  • GMRS Callsign:WQYM541
  • Ham Callsign:KB0HAW

Posted 23 July 2020 - 12:37 AM

I wish we all lived closer.  I would love to sit down and chat with many of you.  There is a lot of knowledge and experience in this group.  I would definitely bring some doughnuts.

Back in the old days, we used to have "CB Jamboree" events out here in the midwest that would bring folks in campers & RVs from across the nation.  Perhaps one of these days, (not soon due to Kung Flu) but one of these days someone could host a "GMRS Jamboree" for the same purpose.... get together for fun, food, and fellowship - and have a big swap meet to swap radios, antennas, parts, and bushel baskets of bull.. uh, chips.


  • n4gix, marcspaz, Mikeam and 1 other like this

#17 WRFV510

WRFV510

    Advanced Member

  • Members
  • PipPipPip
  • 46 posts
  • GMRS Callsign:WRFV510
  • Ham Callsign:none

Posted 23 July 2020 - 02:52 AM

So it would be a good idea that when tuning a antenna to check your lowest, middle, and highest freq and if 1s SWR starts to rise stop.  



#18 marcspaz

marcspaz

    Advanced Member

  • Members
  • PipPipPip
  • 663 posts
  • LocationLocation Location

Posted 23 July 2020 - 08:16 AM

So it would be a good idea that when tuning a antenna to check your lowest, middle, and highest freq and if 1s SWR starts to rise stop.  

 

There are a couple of techniques.  I personally tune for the center frequency of the band I plan on using.  To be sure you are centered, you check the highest frequency you will use and the lowest frequency you will use.  If the SWR is the same on both, you are good. 

 

If the SWR on the high frequency is higher than on the low frequency, your antenna is too long.  If the SWR on the low frequency is higher than on the high frequency, your antenna is too short.

 

The biggest problems you will run into with GMRS antenna tuning is, unless you have an antenna analyzer, that method is tough to use since you can't tune to the center frequency.  Also, in centimeter radio, if you trim the antenna as little as 1 millimeter, you can drastically change the SWR.  So, its a slow process and easy to mess up an antenna.


  • Mikeam and fremont like this

#19 berkinet

berkinet

    Senior expert on absolutely nothing

  • Members
  • PipPipPip
  • 810 posts
  • GMRS Callsign:WQYR510
  • Ham Callsign:WB6TAE

Posted 23 July 2020 - 09:14 AM

...If the SWR on the high frequency is higher than on the low frequency, your antenna is too long. If the SWR on the low frequency is higher than on the high frequency, your antenna is too short.
...

And, if the SWR on both is 2:1 or lower, seal everything up put away your tools and leave it. If you keep trying to “fix” it you will end up with an antenna for the 100 gigahertz band.
  • Radioguy7268, Lscott, WRAM373 - Richard and 3 others like this

Everything we hear is an opinion, not a fact. Everything we see is a perspective, not the truth.

-- Marcus Aurelius


#20 n4gix

n4gix

    Advanced Member

  • Members
  • PipPipPip
  • 628 posts
  • GMRS Callsign:WQWU626
  • Ham Callsign:N4GIX

Posted 23 July 2020 - 01:39 PM

Back in the old days, we used to have "CB Jamboree" events out here in the midwest that would bring folks in campers & RVs from across the nation.


That brings to mind the "Coffee Breaks" my local CB club would organize on holidays. We'd set up at an Interstate rest stop, or perhaps a local mall parking lot on a major US Highway, and serve coffee, water and donuts to weary travelers.
  • Jones, marcspaz and Mikeam like this




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users