Make a 1/4 Wavelength Antenna + Calculations

Today I’m going to show you how to make a 1/4 wavelength antenna for the 70cm amateur radio band. Of course physics doesn’t care who’s using her laws, so this design can be applied to really any frequency, though the lower in frequency you go, the bigger it gets! We’ll cover a little basic antenna theory, build it, install it in the attic and do some testing & tuning on a nanoVNA vector network analyzer.
**NOTE: If you are going to make one of these, the distance from the center of the SO239 to the edge of the metal plate MUST be considered as part of the radial system. So you MUST subtract this length from the wire radials when cutting them.
Sheet metal brake/shear used:
Harbor Freight discontinued it, but looks like it's been rebranded - albeit with an unfortunate name
www.amazon.com/KAKA-Mini-Shea...
Parts used in this build
3/4” OD PVC pipe
RCA antenna mount PN VH123R
1/8” Thick 6061 aluminum baseplate, 4” x 4”
4 Ground radials - Hobart 3/32” brass brazing rods
Larger 5/32” (0.014” wall) center brass radiating element - K&S Precision #1146
Smaller 1/8” (0.014” wall) (0.014” wall)telescoping center brass radiating element - K&S Precision #1145
Stainless 1” x 0.025” strap used for radial “holders” - K&S Precision #87167
12 4-40 screws
SO239 Connector
Follow more stuff here:
@tcurrentsource
www.thecurrentsource.com

[@homoevolutus]

Super excited to watch this video. It took me a couple of hours with all the extra references I had to look up and (of course) all the shopping! I'm taking Intro to Communications in a couple of weeks and am trying to get an understanding of the subject before I do. I hope walking into class with my 1/4 wave antenna isn't too much. Thanks again! (subscribed)

[@americaswayout4489]

Nice to find your channel, again kind of jeolous of all the tools so many have to make home brew stuff. I subscribed so will be following further postings. Cal, havving you and Mike on was a nice surprise this morning.

[@grumpyken9151]

great explanation! very helpful for building 1/4 wave ground plane antennas across all bands

[@sholland88862]

Thanks really helpful. Just studying for my licence in Canada (basic) and reading the chapter on antennas. Nice to get a visual to help it all make sense.

[@jj-js5sx]

You pronounce it the way everyone I know does. Soder (silent "l")... here in MD.

[@Capitaine.Albator]

Amazing video, great quality. Very informative. Thanks for sharing. Subscribed 👍🏻

[@davethewelder1]

That was very enlightening. Like to see more of your videos.

[@pigroper5]

I appreciate the video and design, just got my license along with my dad. We can't quit reach each other with handheld radio antennas. This looks like the ticket. Also didn't realize I could mount an antenna inside the attic. I really like that info too. I was trying to figure out were to mount an antenna outside somewhere that didn't look like junk on the outside. Good stuff. Now I'm figuring out connectors for coax. Little details I got to learn. -Like does a so-239 connect to a pl-259.coax connector, then a SMA female connector that looks like a male connector to the radio. --LOL, Just scratching the surface.

[@danekouttron8768]

woah nanovna looks really neat!

[@SkyGravity1]

Yay, more rf stuff coming !

[@ristomatti]

Thanks! I've always been frustrated with antenna instructions leaving out the details of the build like you mentioned. I seem to lack the imagination/experience to come up with these kinds of methods demonstrated...

[@jeremyemery1438]

Great video! I’m working on building a dualband GP to use on my simplex repeater

[@BlueDawn1776]

I liked your teaching method. Thanks for the good video.

[@dsmith004]

Just found your channel. I find the math helps me understand the design and modifications of an antenna.

[@RC-Heli835]

I like it man! Thanks for sharing!

[@TheGmr140]

amazing video great stuff. i need to get one of those nano vna, i think they now make a tiny spectrum analyzer too. great test tools for under 100 bucks.

[@keithtreisch6890]

Love the video. I have been doing experumenting with j poles fopper and ed fond pvc style. Just built a quarter wave and i love it. I ned to revise how i did mine a little but love the video. Straight to the point and simpke.

[@Chickendoc]

Sweeeeeet build and video - give us another helping!

[@donaldsmith3048]

Thank You for the information.

[@stgodbout]

I'm in the woods here, Laval Que, tree haven, where I'm trying to simply catch 2 FM stations, CHOM 97.7 and The BUZZ 99.9 with a cheapy FM audio receiver/player in my man cave without the shitless static I'm getting so I'll try this antenna out, thanks. After success I'll try the same for the Tele,TV... free over the air signals. Bell and Videotron, over here, is costing way to much $ :)

[@dmgx]

Great info. Well explained.

[@AmRadPodcast]

Thanks!

[@deuce-way1440]

Thanks for sharing. The RF community doesn’t like to share in-depth knowledge these days. So this is a big help🙏🏾🤣

[@NiHaoMike64]

In the US, for VHF/UHF, make the antenna double as a TV antenna since by law, HOAs cannot ban or unreasonably impede the installation of an outdoor TV antenna.

[@AmRadPodcast]

This is true. Sounds like a good topic.

[@Texas1FlyBoy]

TV or FM radio. We use FM for VHF/UHF. Works for me! :)

[@moguy1973]

It's the OTARD law, it now allows for any antenna to be mounted and the HOA can't do anything about it. Not sure if it's worth the fight, but they legally can't do anything to you.

[@electronic7979]

Very good 👍

[@roadkisserful]

Very nice video 🙂

[@BritishBeachcomber]

Just a note about velocity factor. With coax or parallel transmission lines the VF may be as low as 0.5 and metal tube will also be low, if the diameter is large relative to wall thickness.

[@phils2180]

I'm building one of these for portable use using domestic radio telescopic antennas. I found some that extend out to 1.2m so it should be tuneable across the 4m, 2m and 70cm bands. Minimum weight with quick and easy deployment are the main design criteria.👍🙂

[@AmRadPodcast]

Hey Phil. That's a fantastic idea! So in that way you can adjust the radiator.. are you adjusting the radials as well to match? I imagine that you'd have to mark them somehow so you don't need to constantly tune when you change bands? Very cool

[@phils2180]

@@AmRadPodcast The antenna uses 5 telescopic elements, one radiator and 4 ground plane elements. They come with F connectors which I should be able to modify and use so they can be unscrewed and easily stashed in a back pack. A bit of field testing with an analyser or VNA will give me the optimum lengths of each element, then I'll mark them up with a permanent marker pen so setting up should be a 3-4 minute job👍

[@markreed2643]

totaly got it watched it a few times loved the metric vid was great and much apprec M down under auzzie

[@lmp2208]

Thanks for technical explanation. Much appreciated. However, no doubt you realise by now (3 years ago ?) that you slipped a decimal place in your calculations. At 8:00, up pops the line with 1/4 wave length calculation. 0.172 metres should be 17.2 cm …. they don’t cal it “centi” for nothing 🙂 ….. or you could have left it as mm. Assume you’re USA, so maybe you’re use to using Imperial units, and are metrically “challenged”. 😆 I’m from Australia. I read down thru the first 20 comments or so, on my iPhone and couldn’t see any pick-up on this point so posting. I notice one commentator was Canadian (metric), but no mention ?

[@lmp2208]

Ok … I should have kept watching … corrected. Perhaps you should have subtitled the correction immediately instead of waiting 45 secs in the video, and then only leaving it up for 10 secs.

[@stephenwalters9891]

Considering how much space you have in your attic you could install a dual band slim-jim or a 7/8 vertical or a j-pole. at 70cms you can get some serious gain from a vertical G7VFY

[@nidhi_singh9494]

Sir I found your video very useful..😊 I need to design conical monopole antenna (Quarter wave). In the UHF range.. I can't find mostly literature on this topic..can u tell me how to decide length and aperture diameter for this.

[@jluke6861]

Thank you.

[@AmRadPodcast]

You're welcome. Hope it was helpful! -Derek

[@jluke6861]

@@AmRadPodcast Very Helpful. Well explained. Thank you for putting the time on this.

[@DonzLockz]

Great video, well explained and good construction. I'll save this video so I can build one like it.🍻🤠73

[@AmRadPodcast]

Thanks Don! Good luck!

[@doubleD_K2DRD]

Really appreciate your video. Built this antenna for GMRS. It’s resonate at 463800 mhz. Checked it with a NanoVNA and the impedance is very low, around 14 ohms. Any suggestions as to why.

[@charleshills8540]

Great video but I could use more reference literature as I’m only just starting out. Never did this before. Was that the Ventures ?

[@Somedudeontheinternet3202]

Hello, thank you for this video which did clear up some things. It is my first video from your channel and found it very interesting. I was wondering on how to make an antenna in the 320 Mhz range, and so if the rods would indeed have to be shortened.

[@AmRadPodcast]

I found this site that does all the math: m0ukd.com/calculators/quarter-wave-ground-plane-antenna-calculator/ Keeping the velocity factor that they have as default and using my construction techniques, you should be good to go!

[@Somedudeontheinternet3202]

@@AmRadPodcast thank you very much

[@stevie3011]

What is the software you used to calibrate towards the end of the video ? Does it come as part of the NanoVNA analyzer ?

[@eons_of_ions8651]

Would you mind linking to that nanoVNA? Looks pretty sweet.

[@longrider42]

Question, and I hope you will answer me. I have a Diamond 7900 Super Gainer 7/8 wave 2m/70cm antenna, which says on the site where I bought it "Does not require ground plane" But I am planning on using this antenna for Field day, on a 12 foot tall mast. What I want to know, is if I added a ground plane to it, would it help? And if so how long do I need to make the radials. I have been trying to find an answer to this, with no luck. I'm lucky, I live in a HOA controlled mobile home park. But one of the board member is also a ham. So I had no problem putting up a Arrow Open Stub J-pole, on a 15 foot mast. Hey you never know till you ask.

[@sopoelectrician]

Pardon my ignorance but I'm trying to find out how to make a feild expedient antenna that I can just put In a tree vertically or horizontal as need be to hit repeaters while hiking. Thoughts suggestions? Enjoyed the video nicely done. Portland Maine.

[@AmRadPodcast]

Jeffrey Berry not sure what frequency you’re using, but I assume 2m to 70cm.. you could make this same antenna, but using copper wire instead of brazing rod and an SO-239 connector. String the driven element to a tree limb and you could stake a few radials to the ground using same string. It’ll look like a hack, but it’ll work just as well- and no impedance transformation needed.

[@bighangki]

Interesting . Is that possible to make 1/4 lamda groundplane antenna with 1 passing coil? Sorri about my poor english😁🙏

[@mrderekbarthow5967]

For your four ground plane radials is the measurement from the edge of the aluminium base? Or do you measure from the edge of the So239 socket? They are significantly different numbers. Thanks Derek ZL1DCB

[@AmRadPodcast]

The aluminum plate is part of the ground plane, so should be included.

[@TheSlinq]

haha wondered what you meant when you said "I understand the meaning's a little different in the UK"... realised you meant it sounds like "sod her". Yeah.. we say that a lot here, "sod it", "sod them" etc..

[@digitalradiohacker]

At around the 9:14 point, you begin to talk about the velocity factor of certain materials. For coax, the Vf is on the product data-sheet, but would you be able to share your source for the Vf of other materials such as copper? Also, the multiplication for the radials - You said something about "the test book". Is that some part of the US amateur radio exam? Did hear "test book" correctly? The reason I'm asking is that I'm considering doing some videos in the future about antenna construction, and while I can model various antennas in MATLAB, it doesn't show me WHERE it got its numbers from - It just spits them out, which isn't very satisfying. I'd like to cover high gain antennas VS unity gain antennas. Thanks for the videos!

[@AmRadPodcast]

Thanks for watching, and the comment. Yeah, I realized I fudged what I was saying about "test book"; what I was *meaning* to say was "the text books" - various antenna books that I own. Should have clarified, but after being in the 100deg attic for half an hour... proper editing was the last thing on my mind. :) The Vf of a bare wire is peppered here and there in ARRL literature as 95%, though I'm sure varies a few percent depending on the dielectric (air). Honestly, you have to trim an antenna anyway and if you're off one percent for 70cm, maybe we're talking about a mm or two "error". Just constructing the mechanical components, you'll probably end up causing larger errors than that. All of this tolerance stackup will cause you to end up tuning the antenna anyway. Of course as you go up in frequency, you have to be more precise. 70cm, this is all not so critical for general applications.

[@digitalradiohacker]

@@AmRadPodcast Thanks. As usual, Google is our friend, so I'll do a bit of my own research also - I just wondered where your numbers came from. I actually work for a commercial radio supply company doing programming and site instals. When we fit an antenna to a vehicle using anything like the NMO mount, we have no choice about the "radials" because the groundplane is just the vehicle roof. This leaves us with the vertical section to adjust. I've been meaning to try some experiments with vertical length vs coax length to see if there is any kind of mathematical relationship that would get me pretty close. Homeowners association: Antennas maybe specifically banned, but isn't almost your DUTY as an American to put up a US flag? The pole could be fibreglass, and it would be YOUR business what was inside it.

[@roshanantony7467]

whats the function of the radials in the QW antenna u made? Cuz you connected the coax to the center long one right, what happens if you made the antenna without its radials? (noob here)

[@AmRadPodcast]

Good question. If you look at a dipole antenna, which are two 1/4 wavelength radiating elements, separated electrically at the feedpoint.. both elements work in tandem (or against each other) to propagate the RF radiation into free space. For a quarter wavelength antenna, one radiating element still exists, just like the dipole, but it still needs a "ground" to work against. In this case, the radials act as a counterpoise ground and serve as the "other side" of the dipole - at least that's how I like to think of it as. If the radials are 90 degrees to the radiating element the impedance of the antenna doesn't match the typical 50 ohm transmission line and quite a bit of energy from the transmitter is reflected back to the radio. If we droop the radials approximately 45 degrees downward, the impedance more closely matches 50 ohms and the majority of the RF is absorbed/transmitted by the antenna - minimal reflection. If we have NO radials, then the antenna will be severely mismatched and the SWR will be very high and nearly all of the RF will be reflected back to the transmitter.

[@heliumnetwork2871]

I made one for helium miner for 868mhz, now im in testing

[@eagleviewhd]

are the radials 183 cm total lengtth, before being bent?

[@mattmoreira210]

10:08 183 *centimeters?!* Now that's a heck of an antenna! lmao

[@furonwarrior]

I heard adding more radials make it higher gain and a better performing antenna.

[@AmRadPodcast]

For VHF/UHF only 4 are needed. For HF, the more radials, the better… check out my recent HF vertical video for an explanation as to why.

[@andrewverran3498]

Are you still doing a video on tge NanoVNA? 73s

[@AmRadPodcast]

Possibly in the future. Alan W2AEW seems to be doing a series of videos on the NanoVNA. I have to admit, he's done a wonderful job and I'm not sure I could do better!

[@Johnyrocket70]

FCC says any antenna under 3 meter diameter is allowed on every roof. HOA or no HOA

[@lostinkansasonasunnyday305]

.172 meters is not 172 cm, it is 17.2 cm = 6.78 inches

[@happysoul941]

Seen so many vids that cover the subject like the dress of a pretty woman. Enough to see but a lot left to imagination. Thanks for this excellent++ video/tutorial.

[@80lab38]

thumbs up for reverse umlauts!

[@Mojorizen1962]

Will this work for cb radio?

[@AmRadPodcast]

Yes, the theory still applies, but at that frequency the elements would be around 9 feet long. You're probably better off watching my other video on 1/4 wave antenna for HF. Maybe even a bottom-loaded vertical like a K40 and some ground radials would work.. never tried though.

[@56pavel53]

👍

[@w.rustylane5650]

What frequency are you building for? Looks like 2 meter. I just passed my Technician license test yesterday and got a free Yaesu FT-4VR mono band 2 meter HT for passing the test. As I wait for my license to appear, I need to build an antenna that I can hit the local repeater without going out on the deck to do so. I want to be able to hit the repeater from inside my QTH. On the return loss can't ya just use a mirror to reflect it back to the antenna? Just kidding. I thought this build was for a 2 meter antenna???

[@seniorchief48]

KF0OIY….. “*@#’&&*)(“ you made me understand antenna math.

[@Fosgen]

I take copper wire about 1.6-2 mm, make a loop, put into drill and twist. It becomes dead straight and stiff. I am cheap, thats all.

[@mrkattm]

When did you get your ticket ? You have skills and you have some nice equipment but you are still playing around with repeaters?

[@AmRadPodcast]

Thanks. Got my tech back in 1997. When I was working at Texas Instruments, they had (still?) a radio club and repeater across the street. That got me into it. Finally upgraded last year to General. What's wrong with repeaters?! I'll admit I've been asleep at the wheel and have no idea what technology exists now.

[@Texas1FlyBoy]

@@AmRadPodcast Definitely nothing wrong with repeaters. They serve a useful purpose for local communications. Now that you have your General ticket, it does open more doors (frequencies) for HF and it does get more fun there. Congrats and Enjoy!

[@E57det7I]

It is a violation of FCC regulation for a homeowners association to require or attempt to stop a resident building and operating ham radio, and all of its required instruments, including antennas. It is considered an emergency communication method, and any local authorities that attempt to prevent a licensed ham operator from operating are in open violation of federal law. Regardless of any HOA agreements.

[@jamesvargas5011]

Free Space Impedance = 377 ohms.

[@tfaber9394]

Great awesome info. Thanks Dave. Km4vgo. 73. Hey u tubr Rangers, Hit the like button guys. It's more important than you think.

[@krisraps]

So Sad, Own A Property But You Can`t UIse An Open Antenna. My Yard Is FULL With Antennas And No Dog Is Barking

[@AmRadPodcast]

I’ll be putting up a (hopefully stealth) 20m/40m vertical in an upcoming video. Should be fun

[@billkabler9545]

Amateur radio antennas are exempt from H.O.A. Rules.

[@LeeMcc_KI5YPR]

Old comment, but no, ham antennas must follow HOA rules. There was a bill in Congress to change that, but it died at the end of the last session. There is an agreement between the ARRL and the national HOA association for HOAs to make reasonable accommodations, but no HOA is required to gallows it.

[@SimonsRadioSpam]

I think you mean millimetres not centimetres 😬

[@jasoncreed6396]

Soso demo but I think I'm scared for life by that there goatee ;).Take some advice from someone with impeccable taste...loose it, lol

[@johnwest7993]

As for the word 'solder', the 'L' is silent the way I pronounce it. And since I'm in my 70's I tell people it's been pronounced with a silent 'L' all my life and since before they were born. They should get used to it. :)

[@mrderekbarthow5967]

In New Zealand it's been pronounced with an "L" for the past 70 years. I thought Soder was a place in the Thomas the Tank Engine stories we read to the grandchildren.

[@JohnSmith-wl8cv]

Why do Americans leave out the ketter T in inter they say inernet 😂 mobile not moble solder not soder

[@mariannevanoostenbrugge3541]

you are talking 172mm , and one take later, it is about 172 cm, i lost you here

[@AmRadPodcast]

So sorry. I meant meters. Good luck on your build!

[@JohnSmith-wl8cv]

There are online antenna calculators to work out the measurement of antennas ,just go tell us the measurements, never mind the science lesson get on with ,,I skipped to half the video

[@matthiasBdot]

Hoa.. i love the reddit stories of these karen regimes..

[@matthiasBdot]

Build a karen jammer

[@sandhill9313]

There is no L in the American pronunciation of solder. As all of your other pronunciation is consistent with American English, I don't know why you would be compelled to talk like a ferner for that word 🙂

[@TheSlinq]

haha wondered what you meant when you said "I understand the meaning's a little different in the UK"... realised you meant it sounds like "sod her". Yeah.. we say that a lot here, "sod it", "sod them" etc..