Introduction to Quarter Wave Antennas and SWR Relationship on the FT991A
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- เผยแพร่เมื่อ 11 ต.ค. 2024
- This video is a collection of learning notes and materials I been collecting from various You Tube Channels and books. The focus is to share with newer HAM’s some of the things I find interesting and to encourage more interests into the various topics by sharing my personal experience.
I have learned and am starting to grasp the world of RF and how things work with a focus on Antennas and Feed Lines. I find this very interesting and with the knowledge on the internet will allow HAM’s to build their own antennas or at least know what to look for when shopping to obtain the best antenna delivery system.
The purpose of the video is to review basic ¼ wave Antenna’s and some of the characteristics of their feed point impedance and how this interacts with the SWR on your FT991A
There are a wide variety of ¼ wave Antenna’s covering 70cm all the way up to 160 meters.
Some of the ground planes antennas for 20 through 160 meters are so large that they require to be mounted on the ground surface and use a base plate with screw holes where you attached physical wires that act as a ground plane.
We also look at the vertical ¼ wave and the horizontal ½ wave Dipole Antenna and see the relationship as to orientation and some common observations I found that were interesting.
The purpose is to show why a relationship between the 50 Ohm impedance of the FT991A radio antenna connection should match as close as possible to a resistive impendence match of the antenna feedline system.
With a good impedance match there will be a minimal reactance (capacitance and inductance) in the feed line system. The result is lowest SWR possible and a very happy transmission feedline and radio.
The antenna Vertical Radiator length is typically ¼ wavelength, there can be cases where the vertical can be 5/8 wave vertical is used and others, the ¼ wavelength is the most common. The ¼ wave is a self-resonant link which makes it a popular choice.
The ¼ wavelength vertical antenna can have up to 3 to 4 Ground Radials, each at a ¼ wavelength mounted at the base of the vertical and are horizontal or sloped down at an angle, depending on the ground radials angle will determine the antenna feed point impendence, these radials act as ground surface to radiate the signal in all directions when the base of the antenna is at or close to ¼ wavelength height or more above the surface of the earth.
The ¼ ground plane antenna for example 20 to 80 meters can also be mounted directly on the ground, use of Ground Wire Radials along the ground typically are buried just below the surface, I have seen many examples where up to 16 wire radials and are common.
The length of the buried ground radial wires is not restricted to the ¼ wavelength length as shown in previous slides on radial at the base of the vertical ¼ wavelength so you can have different lengths of wire radials buried just below the ground surface. A common connection point is a steel plate that also supports the vertical radiator.
The below shows the formula to calculate the length of a ¼ wave ground plane radiator for use on the 2-meter band.
234 ÷ 27.335 MHz = 8.56’
8.56’ x 12” = 102.7”
Links:
Arrow Antennas 2m, 6m, 70cm
www.arrowanten...
Hustler Antenna G-2537 DXE Engineering Web Link:
www.dxengineer...
COMTEK COM-20VA DXE Engineering Web Link:
www.dxengineer...
DXE-40VE-1 DXE Engineering Web Link:
www.dxengineer...
DXE-7580FS-VA-1 DXE Engineering Web Link:
www.dxengineer...
DXE-160VA-1 DXE Engineering Web Link:
www.dxengineer...
DXE-VA-PIVOT-3 - DX DXE Engineering Web Link:
www.dxengineer...
DXE-RADP-3 DXE Engineering Web Link:
www.dxengineer...
'73 from KI5JUF
Enjoyed watching this. Thank you for taking the time to put this together. 73
Good evening sir and thanks for the message 73 keep in touch KI5JUF
Nice Informative video....When you can't or don't want to angle Down the Ground Plane Radials but Want a perfect 1.00 SWR, You can approx 5% Shorten the Vertical Radiator & connect a Beta Hairpin match Across the antenna Terminals....Adjust the Vertical Radiator & Beta match shorting strap as usual to Tune a perfect 1.00 SWR......
Good evening Tom, thanks very much for the message and the tip, I've had others tell me the 5% shorter on the radials too, the Beta Hairpin match is cool, did not know that so that's good to know, appreciate the message and the knowledge thanks again from KI5JUF '73
Excellent training material that gives great insight into a very important topic for ham radio. Thanks for providing this material. 73 KD4FJ
Welcome to the wonderful rabbit hole that is antennas ; )
Rule 1 Any antenna is better than no antenna Then the rabbit hole opens up and its a deep one
9:40 Yes, you are correct. Going the other way, if you slowly raise the ground plane until it shorts-out the driven element, you find the zero-ohms end. LOL... ;-) This is all very well known, since the early 1900s (so I've read, I'm not THAT old, LOL!!).
I see you did a lot of research. Good job. I do have a couple things you may want to think about that are not widely known. One is that with a quarter wave vertical as you explain you need the other half of the antenna. With above ground radial they do need to be a quarter wave long, but there is no need for the radials to be a quarter wave long or the same length as the vertical portion. Think of an off center fed dipole. Moving the feed point off center simply increases the feed point impedance. As long as the overall antenna is resonant the vertical plus the radial length needs to add up to 1/2 wave. I will go one step further and say it does not have to be resonant as long as you match the impedances. It is easier if the system is resonant. With radials on the ground you are making a capacitive coupling with the ground. You need a low impedance connection to ground and that connection can be low resistance ( several ground rods) or low reactance (Xc). You want low impedance (Z). Z = R +jX so small R or small X either one will give low Z. The return currents will flow through R ( low R is less loss) or they will flow through X. In this case Xc. Large Capacitance means low Xc. However studying for the Extra Class you should run across the complete formula is Xc = 1/ (2 pi F C) so the reactance depends not only on C but the frequency as well.
One thing to always remember in electrical circuits, whether it be AC, DC, or RF) is “ current always returns to the source”. With RF the source point is usually the SO 239 connector on the radio. So any current leaving the center pin must find a complete circuit back to the “ground side “ of the SO 239. That path is usually up the center conductor of the coax to one side of the dipole or the vertical element. The through the air and back to the other side of the dipole where it flows down the inside of the coax shield to the “ground side” of the SO 239. Here ground means chassis ground not earth ground. With the vertical and radials, the return current leaves the vertical and returns through earth to the radials where it flows through the capacitor formed by the radials and the earth, then to the coax shield and back to the source ( the ground side of the SO 239 on the radio.
With no radials the return currents will try and find some way to get back to the source. It will not follow the path of least resistance. It will follow any and all paths that might exist. The largest current will follow the path of least resistance but there be some current that follows other paths also. Like current division in resistors when you have several resistors of different values, the current divides up in proportion to the resistance. Maybe I should say in inverse proportion to the resistance! I always recommend the ARRL antenna book ( you can’t have one too old either) and the ON4UN Low Band DXing book. It has a great section on antennas that applies to all bands not just 160 meters! There is a lot of miss information on You Tube so having multiple sources of reputable information to check against is extremely important. I am a retired EE and also have several old text books in addition to what I mentioned above. It’s a complicated subject with a lot of myths people believe as the truth. You did a good job keep up the good work!
Forgot to mention that a 1:1 SWR is not necessary. Total loss in dB is what is important. Sometimes a high SWR only adds negligible loss and sometimes it does not. It all depends on the matched loss of the feedline to start with.
Good evening sir and thanks for the message I really appreciate the feedback I will study this information this will also give other folks some good information 73 and thanks again de KI5JUF
Your middle Vertical antenna at 10.00 has been built for use on 2m and 70cm bands (18 5/8” and 6.25” radiators and reflectors) and verified via NANOVNA to produce an SWR less than 1.1:1 on the 2 m band. The Smith Chart shows 50 +/- ohms which confirms the low SWR. Suggest you may want to review your theory about characteristic impedance/resistance of 73 ohms with this configuration. May help you succeeding on your General or Extra licenses. Best luck to you. Dave
Thanks for the info - this helps. I'm also a general who is planning on testing for extra next month.
Hi Scott, glad the video was helpful, let me know how you do on the test, do you have a You Tube channel series your watching? I found the HAM Study app and the You Tube series from W4EEY covers Technician, General and Amateur Extra . Anyway, I was curious, thanks for the message always love to hear from folks and '73, I usually work 10 meter USB, FM so if you one night maybe we can make a QSO!!
Hi Scott, just checking in did you have a chance to test for Extra, I tested yesterday and got the ticket, hope all is good and best of luck on the test '73 KI5JUF
@@westtexasvideogateski5juf255 Awesome. Congratulations. I just today signed up for testing on August 20th, so I hope I do as well as you did. I'll let you know in a couple of weeks.
@@westtexasvideogateski5juf255 Hi, I tested and passed the Extra test today, so I'm a happy camper. Next on my list is to improve my HF antenna set up (currently EFHW in the attic due to HOA). Probably try either a EFHW or Vertical outside and ask for forgiveness, rather than permission, if it becomes an issue. After that, toying with the idea of learning CW....
@@scottb.2022 Congratulation's Scott that's great let me know how the journey continues and have a great week, let me know if you need anything from me '73 sir
How can you get 45 degree radial if feed point is 6’ up and radial is 16’ long or longer? Thanks great video.
10:20 The current into both elements will be basically the same (varying only due to environmental asymmetries). Even though one dipole element is connected to the shield, and the other to the 'hot' center conductor. That said, I still put the 'hot' element up higher, more on superstition than logic. But don't think for one second that the tip of lower element won't give you a shock !! There's roughly equal voltage and current in both elements.
Traditionally, the elevated ground plane radials could be about 5% longer than the vertical element. When I say "Traditionally", this was the rule of thumb circa late-1970s.
when you look at it. at least to my way of thinking,when you have a vertical quarter-wave with a single wire the same length it will end-up making a half-wave dipole be it straight down or at an angle like an inverted L
KO6DEV- why is my 1/4 wavelength antenna resonating at full length?
The angled ground plane, in addition to raising the feed point impedance to a perfect 50-ohms, is also supposed to slightly lower the angle of radiation more towards the horizon, thus hopefully improving the signal for the most-distant stations. So the description of the signal bouncing up (implying a higher angle of radiation) is not as I've read elsewhere for so many decades. The antenna should be viewed as an assembly, not conceptualizing the signal bouncing off the ground plane. The wavelength of the signal is bigger than the antenna, so the signal doesn't treat the antenna like individual parts.
Good evening sir, thanks for the message and tips, I'm carefully diving into the world of antennas, I just recently passed my Extra and that has peak my interest in really trying to understand the whole HAM experience, I have been watching a ton of videos and practicing all the math associated with the Extra. thanks again great advice and keep in touch I appreciate all the comments helps me grow in the hobby '73 sir from KI5JUF
why is my 1/4 wavelength antenna resonating at full length?
10:37 I realize that's a 2D representation of a 3D shape, but it's... , er, ah, hmmm... Should be a 3D doughnut shape that puts the peak generally towards the horizon (all around in azimuth), but up a bit in elevation (not extremely low angle as it's not the world's best DX antenna).
I'm not a ham, but don't get confused with resonance and SWR. You state, 'with a good impedance match there will be minimal reactance (capacitance and inductance) in the antenna feed line system.' This sentence has mixed theory and confused terms, like, in the antenna feed line system.
In regard to a 'good impedance' this would be referencing the antenna resonance, the resultant. I would suggest you research what actually makes an antenna resonant. Many hams simply say it is antenna that is purely resistive and exhibits no reactance. That is not the definition, that is the outcome, they are often skipping an educational step, and one of the major fundamentals of RF theory. Why would one do that? I mean, get a real understanding, what's stopping you?
So, we now have the antenna on or near resonance, but the theoretical quarter wave antenna over a perfect ground has an impedance of 36ohms, and our radio and feed line is designed for 50. We must move the ground away, in this case an elevated ground radial system, to bring it up to 50ohms. In other words, they are separate. Sure, you can shorten or lengthen the driven element to change the impedance for what ever reason necessary, but don't get the fundamentals confused, that's all.
Good evening Jeremy, I really appreciate the message this is the kind of feedback I love, I've had to learn most everything on the Internet and reading and I'm always concern I might be putting bad information out, again thanks so much your feedback is most appreciate that sir keep in touch and thanks again!
Slides 2.5 and 2.6 show 50-ohms divided by 50-ohms = 1.1, which is not correct. It should be 1.0:1, or 1:1, or 1 (however you wish to write it). Not 10% off at "1.1".
8:40 Also narration, "...down to 1.1..." Should be '...down to One-To-One (1:1)..." We're getting the decimal point "." conflated with the ":".
Copy that thanks