Building a better End Fed Half Wave Antenna for Ham Radio?

all right folks so I want to talk a little bit today about nfed halfwave antennas and this is one that I was working on today and this is pretty goofy looking I get that um so I want to talk a little bit about how I got here and um how this thing came to be so if you take a look at it there's a couple of different things that are going on here and the first one is that we have 14 turns of a secondary here in this enamel copper wire and uh just 14 wraps around the toid and then for our primary we have a double wrap of this 18 gauge stranded uh silver coated silver plated I don't know um copper wire and that is connected to our primary and our secondary and then up here we have a capacitor but we we'll get to that in a little bit when we take a look at kind of some of the more standard builds of an nfed halfwave what you have here is your primary is two wires Twisted which also includes the first uh two wraps of your second secondary and we'll look at a schematic picture so you'll be able to see this um then your one end of your primary connects to your Center conductor and your coax the other end connects to the coax will shield in the ground and then it also runs down to this counter Poise connection and then we have across our connection here we have this 100 peaf farad capacitor now this is a very very early version of the Cenna Apollo and this was actually called an emis was the first first antenna design that we did and it works yeah so I keep it around it's a little rudimentary and it's a little basic before we built these um this was actually the first prototype of an antenna we buil in a 3D printed case and you can see it's the same Twisted primary with the secondary cap uh shorting well I guess the Cap's going from Shield to conductor and then we have the conductor running to a counter Poise or ground and then the end of the antenna connects to this lug just like it does on this one here so how did we get to this design let me pull up some uh some stuff real quick and we'll talk through it so here's an article and let me turn myself off so it's a little bit easier to see but this kind of depicts the layout of the original Apollo that I showed you this is a very very common design that is used all over the place when building nfed halfwave antennas and so when we take a look at it you see your 14 turns total on your secondary your primary is this red wire that's Twisted with your secondary you see the connections to ground and you see the 100 pea farad capacitor pretty normal stuff right so I was talking to my buddy Ziggy and he was showing me this antenna that he's working on and this is a transformer for a 36:1 but the concepts are still the same and if you take a look I think it's 12 I didn't count them 12 wraps on his secondary and then the primary being the yellow wire is connected to the center conductor in the shield but there is no connection from from the secondary to the shield and he was telling me about this design and in his testing what he's seeing is a little bit of a lower noise floor and he's seeing less common mode current issues and when he was telling me about this I started thinking about it and I was like you know I've seen stuff that is similar to this and I shared some information with him and let me pull that up so the first thing is is that this is a picture of some antennas from the ra Hamill's antenna book and it's the fuch antenna and I'm not really sure how to say that but that's how we're going to pronounce it now which was um an older antenna that came about in I think it was 1927 is when he patented but heang if you take a look at it on the top part of this antenna you have your transmission line you have a variable capacitor and then you have your inductor and the inductor is completely isolated so it uses something called inductive coupling I believe in order to transfer energy from your transmission line into your antenna which is what is up here where it radiates he has I for current and if you take a look the primary uh part of your inductor is uh to ground but there's no connection between these two pieces of wire now that is a the modern the original form if you come down here look at B which is kind of like what we use these days this is called the modern form and then you can see on the antenna side it is in fact shorted to the coaxil shield and this is a couple of different things so folks will not use counter poises and tend to use the counter Poise as part of your use your coaxis part of your counter Poise and we don't like doing that because we use our counter Poise to kind of balance against the current that is going through our radiator and the way the counter Poise works is is that it's sto it's like a big giant capacitor where it'll store energy momentarily and then dissipate that energy if you feed your ground into your coaxial Shield it searches for ground so it's going to go down your coax into your radio into your hamshack until it finds a suitable ground now a lot of people like well I put a choke in place so that doesn't happen what now well so if you do that you're attenuating your common mode current you're not eliminating your common mode current and you still don't get the capacitive benefit that you do from a real counter Poise so we like to use counter poises but um anyhow I shared this with my buddy and I said hey take a look at this this is kind of like what you're working on there so I also shared this website uh by Steve Yates aa5 TB thanks Steve and in here he talks a little bit about unfed antennas and here's one of the things that he says that you shouldn't do and if you take a look at this this doesn't have the short to the shield and the coax but he doesn't have a counter Poise on here so he's talking all about the benefits of counter poises and when we come down here he kind of talks about the ideal and fed halfway and here you can see a counter Poise um this also says return at 05 of a wavelength long and you'd be surprised how long that actually gets when you're talking about low frequency but the moral of the story is is that and you see in these different ideal infed half waves there's no connection to the shield of the co- axle cable and down here some antenna designs that he does now he's got three wraps on the white which is the primary and then 21 on the secondary but he's using a really really small inductor here and one of the things I noticed on the modern designs when folks are using capacitors they're Capa they're using the capacitor across the primary in this picture he's using the capacitor against the secondary and uh I found that to be a little bit interesting and so when you look down at some of these different diagrams sure enough the capacitor is on the secondary and here he's talking about some of the things that can happen to you if you short these out but uh that's for another com conversation so this is the original design of this antenna that I did so I built this one uh earlier today when you take a look at it up here in the upper leftand corner I've got my element connection and down here I have a um counter Poise connection that I'm going to have none of that is connected to the shield of our Co axle and then you can see I have this capacitor here this is actually a TKD it's like a RF specialized capacitor at 100 paars and I just have that shorting out the connection and shorting is not the right word so I shouldn't use it but I I have that between the center conductor and my co axle shield and that's the way that I've always built nfed halfways and I just thought that that was the right thing to do when I did that here is a chart from nanovna and I just ran a quick Sweep with about a 2500 Ohm capacit resistor across um the the counter Poise or the ground connection and the element connection and that's depicted here in blue and it's really high for an nfet halfwave and I said wait a minute that doesn't sound right that doesn't look right that doesn't smell right so I took the capacitor off and then that was this green line here and then you can see at um 10 meter band where it's a little bit higher our SWR is 1.9 and I was just thinking to myself this doesn't seem right so what I did is is I took the capacitor off and stayed exactly the same nothing changed so then I started thinking about it and I went back and looked at some of those documentations and I said hey man that capacitor is supposed to be across the secondary not across the primary and so that's when I ended up with this design and so I'm had to get a different capacitor that had longer legs and I attached it across the secondary and the green wires they run off to my resistor that I'm using and so when I ran the test there it was still the same nothing nothing had changed so I took off the resistor and again nothing had changed so what I realized is that the nanovna is probably not putting enough current through this green wire to inductively couple with my secondary Transformer so I'm going to have to test it underload but the reason I'm making the video is one is I wanted to say hey this is what we have going on this is the kind of stuff that we go through as we do antenna design and testing but I wanted to see if anybody had any ideas about this any thoughts or any feedback that they wanted to give I know that I get a lot of feedback in the comments section um but I didn't know if I was going to get anything of value about this particular topic today but I figured I reach out to the folks that watch these videos and see if there's any help or Insight or wisdom that can be provided anyhow I think that's it so want to say thanks for watching I really appreciate it uh like comment subscribe share uh if you have any questions post them below thanks

Channel: TheSmokinApe

Views: 5,639

Keywords: #Ham Radio, amateur radio, ham radio basics, thesmokinape, smokinape, end fed half wave antenna, ham radio antennas

Id: odxBEbl8tgM

Length: 9min 54sec (594 seconds)

Published: Fri Mar 08 2024