Why am I a better antenna then my antenna? - Page 1
Forum section: electronics / rf-microwave

author: Beamin
author_rank: Super Contributor
timestamp: December 05, 2019, 01:21:28 pm
content: This always drives me nuts. I adjust the antenna (Live in Victorian brick building in the city(the nice part)) just to get it in the right spot to pick up my favorite classical station or hippie liberal LGBTQRSUV approved NPR station just to walk away and have the reception turn to shit.So I bought one of those big 1/4 wave length 4 element yagi antennas that everyone turns into 2m ham antennas and put it out side. Its mounted vertically which is not ideal but I only can mount it to the fire escape until I get more mounting hardware. Would taking the front two directors and collapsing them and possibly rotating the reflector to do the same make it more omni directional? It connects through 75 ohm coax. The radio is one of those shelf boombox type deals with the telescoping antenna. So the 12'Foot coax comes inside and I tried taking an alligator clip to the center conductor of the coax clipped to the antenna, no improved reception, so I collapsed the telescoping ant., moving it, no luck. Right now just the coax shielding is touching the antenna and it kind of works but I can't get stereo reception. Is the shield of the coax just working a a 12' wire antenna? I can't get outside to adjust the antenna until they fix the fire escape so I'm trying to think about whats going on with this RF voodoo or RFBS as I call it.What makes my body such a good antenna? Is the trick to making metallic objects "connect" to the antenna to use a small nf or pf ceramic cap to get it to couple, somehow blocking undersireableness? Is there any benefit to grounding out the shielding? I can't put a ground rod into the cement outside (have no idea how people in the city get good grounds or maybe they don't?) but I can use either the AC outlet ground or use the fire escape as a ground.How can you test to see if something is working as an RF ground? Haven't even tried to use my shortwave ham radio or even the receiver yet, even though I could stretch a long wire out 40' tall tying one end to the roof from my 2nd floor location.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 05, 2019, 01:35:37 pm
content: ah looking for extra terrestrial radio channel, why dont you stick with Discovery Science?

author: vk6zgo
author_rank: Super Contributor
timestamp: December 05, 2019, 02:50:12 pm
content: Quote from: Beamin on December 05, 2019, 01:21:28 pmThe vertical mounting is not ideal, but many FM stations are circularly polarised to equally service home receivers & car ones, so it won't make that much difference.What will make a difference is the the directivity of your antenna.It needs to point at the FM station antenna.If you can't do that, you may be able to use a reflection from another building or some other elevated object.It may be getting a bit dicey, though, as 100MHz is a bit low for reliable reflections.Yes, you will get reception basically using the lead in cable as an antenna, but it exhibits the "hand capacitance", or rather in your case, "body" capacitance you have observed.

author: Bud
author_rank: Super Contributor
timestamp: December 05, 2019, 04:16:06 pm
content: When you tune an antenna being next to it your body becomes part of the tune. As you walk away after, the antenna becomes detuned. It is tricky in an urban environment. The trick is exactly the opposite than you think, that is - antennas need to be put away from metallic objects to reduce unwanted coupling and maintain the antenna radiation/receive pattern. Try snapping a few ferrite chokes on the coax where it enters the dwelling and play with moving them on the coax withing lambda/4.

author: radiolistener
author_rank: Super Contributor
timestamp: December 05, 2019, 08:09:45 pm
content: Large antenna has large reactive near-field region. It means that noise from any metal pieces and noise sources (such as your fire escape) which falls into this region will be captured and amplified by your antenna.Another issue is that unbalanced antenna leads to common mode currents on the outer surface of the coax cable braid. So, your feeder works as a part of antenna and any metal pieces and noise sources which falls into reactive near-field region around your coax cable will be captured and amplified by your antenna. The cable literally sucks noises out of your house.So, your antenna receive a lot of local noises and useful signals are flooded with high amplified noises from local sources.Solutions:1) Place your antenna as far as possible from metal things on your house (such as fire escape) to reduce influence of the local noise sources on the antenna.2) Use balun and RF choke on the feeder in order to reduce parasitic antenna effect of the coax cable.These metal things like your fire escape works like effective antenna for all noise sources in your house. These local noise sources are SMPS, computers, notebooks, electric stoves with thyristor switching circuits, electric motors, washing machines and other.So, when you place antenna near such metal things on your house, it's the same as connect your receiver to a noise source through capacitorYou can check how noisy your fire escape by connecting receiver input directly to metal parts of your fire escape. You will hear a lot of noises from your home.

author: Beamin
author_rank: Super Contributor
timestamp: December 05, 2019, 09:47:55 pm
content: Quote from: radiolistener on December 05, 2019, 08:09:45 pmHow does a balun work at 100mhz for coax? I have only made them for long wires down on SW. When you say toroid just put them around the coax or try to do one(2 turns/one loop) through it? Doesn't the balun need a good RF ground to work? Would the fire escape work as an RF ground or more like noise generator? They came to fix the fire escape today but forgot the replacement door idiots so more waiting before I can mess with the outside part.

author: TheMG
author_rank: Frequent Contributor
timestamp: December 06, 2019, 03:20:55 am
content: Yagi antennas are directional! You have it pointed up toward the sky, that's not going to do you any good at all. The antenna's boom needs to be pointed toward the source of the signal, with the side with the shortest element toward the signal.Basically because of the improper antenna mounting, and its close proximity to metal objects (antenna elements should be kept well clear of metallic objects to get proper antenna performance, this is why such antennas are usually installed on a vertical mast), the impedance and characteristics of the antenna are all screwed up, and the presence of your body near the antenna messes it up even more, skewing the radiation pattern and making it receive differently.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 06, 2019, 04:30:09 am
content: Quote from: TheMG on December 06, 2019, 03:20:55 amI assumedBeaminmeant vertically polarised, not with the boom looking up at the sky.

author: Bud
author_rank: Super Contributor
timestamp: December 06, 2019, 04:31:48 am
content: Wow you are right, i did not notice that, indeed OP mounted the Yagi vertically !

author: OwO
author_rank: Super Contributor
timestamp: December 06, 2019, 04:45:01 am
content: Might have better luck with magnetic loop antennas at this frequency if space is tight. At VHF you can design magnetic loops that you can put within 1cm of your body and not detune:https://www.eevblog.com/forum/rf-microwave/multi-section-magnetic-loop-antennas-for-low-loss-and-hand-effect/

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: December 06, 2019, 04:59:14 am
content: Quote from: Bud on December 06, 2019, 04:31:48 amAnd installed a 144 MHz antenna to receive ~88-ish MHz* signals?Edit: Maybe not -- he did say "turns into 2m".*NPR stations tend to be at the low end of the band

author: Bud
author_rank: Super Contributor
timestamp: December 06, 2019, 05:11:11 am
content: Anyways, Beamin- you will need to mount the antenna with the elements parallel to the ground and point it at the transmitting station, with the shortest element being at the front. The surrounding may still affect the antenna directivity so some experimenting with antenna azimuth position may be needed.

author: radiolistener
author_rank: Super Contributor
timestamp: December 06, 2019, 05:21:48 am
content: Quote from: Beamin on December 05, 2019, 09:47:55 pmGood counterpoise is needed for your antenna, not for balun. You're needs balun and RF choke to avoid using coax cable as a part of antenna.Quote from: Beamin on December 05, 2019, 09:47:55 pmIt may works good if there is no any kind of electronics in the building, but I suspect there are a lot of electronics. So, it will works more like noise source

author: Beamin
author_rank: Super Contributor
timestamp: December 07, 2019, 12:15:44 am
content: Quote from: Bud on December 06, 2019, 05:11:11 amI had only a few minutes to set up the antenna and cant go back outside until the fix the fire escape. If I get to lay it vertically but it is pointing at a building say the one across the street (on second floor but not on roof level where it has LOS toward the trans ant) will this do any good? I was hoping to point it up and catch signals that bounce off the buildings that are circularly polarized signals to start. It is a commercial FM broadcast band ant. was like 35$ and can be set up with no tools and folds up nicely, really a great deal and you can cut it to any size you want 2m 75cm etc its like its built from an erector toy set. Has some sort of matching transformer where the F plug goes.

author: OwO
author_rank: Super Contributor
timestamp: December 07, 2019, 02:21:07 am
content: Keep in mind a yagi is very sensitive to element lengths and (to a lesser extent) spacings, so if you did modify it make sure you got all the dimensions correct and maybe do a simulation using 4nec2. A yagi is also narrowband so you have to tune it to the station you want to receive (tuning the entire FM broadcast band would require a fractional bandwidth of 22% which I haven't seen achieved by any yagi design).

author: vk6zgo
author_rank: Super Contributor
timestamp: December 07, 2019, 03:33:21 am
content: Quote from: OwO on December 07, 2019, 02:21:07 amCommercially made antennas for receiving FM Broadcasts are broadband by nature, so they aren't normally simple yagis.

author: Beamin
author_rank: Super Contributor
timestamp: December 07, 2019, 11:41:24 am
content: Quote from: vk6zgo on December 07, 2019, 03:33:21 amYes the active element on this is about twice as long as the rest in a () flattened ellipse shape. I don't understand how that works is it half wavelength and the rest of the passive elements are 1/4 wavelength?Its a stellar labs 4 element long range FM antenna, I bet some here have a chopped down one they use for 2 meter.https://www.newark.com/stellar-labs/30-2460/outdoor-fm-antenna-four-element/dp/97W3565If you fold the passives along the length of the beam don't they "disappear" to the antenna and it just becomes a one element dipole and only slightly directional?

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 07, 2019, 01:33:16 pm
content: There are online references and apps on building diy yagi. so you can punch in or tune frequency of interest and see if the calculated lengths match the yagi on hand. How they come about with those lengths and formula, i guess only mr yagi knows how and why.

author: Beamin
author_rank: Super Contributor
timestamp: December 09, 2019, 03:47:43 pm
content: Quote from: Mechatrommer on December 07, 2019, 01:33:16 pmMr Uda knows to but never gets credit.The way I always built yagis was to take the measurements for one with a number of elements that I could physically fit in my house/yard. Then I would take the main element and scale the antenna and all the other lengths and spacing between off that ratio. So my 2m antenna is a miniature version based off a 10 meter 4 element I saw plans for on the internet. That is sound reasoning correct? As all the ratios will always be the same as the wave length gets smaller?

author: shved
author_rank: Regular Contributor
timestamp: December 12, 2019, 04:10:17 pm
content: I'm in some challenging conditions(noise-wise), and i recommend mag-loop.

author: Beamin
author_rank: Super Contributor
timestamp: December 18, 2019, 01:59:45 pm
content: Quote from: shved on December 12, 2019, 04:10:17 pmI was looking into building one. Can you also transmit with them? I have a WSPR on 7m Im building and would like to use it. For transmitting such low wattage is a SWR meter a must? Where do you get the big tuning cap? I don't have access to ham swaps. Never seen those sold at electronic places. Obsolete?

author: hagster
author_rank: Frequent Contributor
timestamp: December 18, 2019, 04:02:42 pm
content: Connecting a single conductor of your coax only works to make the antenna longer. Assuming that the cable is not inline with the antenna it is effectively destroying the antenna pattern.To use your yagi properly you need to remove the built in telescopic antenna, measure the input impedance of the radio, build a matching circuit(if needed) to match your 75ohm tx line and connect you coax between the antenna connection and the RF ground. Otherwise you will never get power efficiently transferred from the coax into the original antenna as the impedance will certainly be nowhere near 75 ohms where your crock clip is.

author: shved
author_rank: Regular Contributor
timestamp: December 18, 2019, 07:01:54 pm
content: Quote from: Beamin on December 18, 2019, 01:59:45 pmYep, you can. Inefficiently, but better than no antenna at all.For serious power you need serious HV capacitor, preferably vacuum. For a start you can take refrigerator copper tubing or coax and double cap from old tube(valve) radio. That's what I've done. If your rig has inbuilt SWR meter(most radios have it but accuracy is questionable) you can use it. If you have nothing, you can tune loop in on receive(maximum sound) that would get you close. Manual tuning is a b***h, some caps have reduction drive - that's what you want. I've got the cap from local flea market for about 2$.I'm now prototyping remote controlled mag-loop antenna for Yaesu FT-857 and some other rigs that support ATAS + some other radio gadgets. Cap in my prototype takes 10w. If i select 20w - arcs follow.Photos of the loop and stuff:https://flic.kr/s/aHsmHPwA1M.

author: fmkit
author_rank: Contributor
timestamp: December 19, 2019, 12:48:01 pm
content: Beamin, try using instruments like NanoVNA or (easiest) Arduino w/ RDA5607 chip to get RSSI readoutOnce you get numbers it's easy to see if antenna too short - your hand in antenna's proximity makes RSSI go up. Using simple antennas ( 15cm tie-wraps) I get almost 1km range w/ only 3dBm power. Here is photo of tx / rx side by side

author: Datman
author_rank: Regular Contributor
timestamp: December 26, 2019, 10:50:59 am
content: Quote from: vk6zgo on December 06, 2019, 04:30:09 amIf you look at the picture, you see that antenna is pointing at the sky!At what distance is the transmitter?Are there powerful signals of other stations at very close frequencies?

author: coppercone2
author_rank: Super Contributor
timestamp: December 26, 2019, 12:03:22 pm
content: is this what occurred during your experiments?

author: coppercone2
author_rank: Super Contributor
timestamp: December 26, 2019, 12:25:28 pm
content: This is a good magnetic loop antenna. I did the same design.https://www.qsl.net/m0ayf/active-loop-receiving-antenna.html#The_loop_amplifier_circuit_diagram_isNot sure how to modify it for high frequency FM, I only used it for low frequencies.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: December 26, 2019, 12:37:21 pm
content: How come you never hear of people with metal plates in their head picking up the local radio station anymore? Are there fewer plates in heads?

author: SeanB
author_rank: Super Contributor
timestamp: December 26, 2019, 04:50:46 pm
content: No more amalgam fillings to act as electrolytic rectifiers any more, almost all have gone to epoxy, and the metal plates are no longer in use, as they have been replaced by less damaging materials, like Dacron fibre and decellularised bone, so that the body will grow a new matrix into the area instead. Only time you get metal is for wire and plate supports for bone regrowth, and titanium alloy used as stubs for teeth. Wire is temporary, removed after a while, and the plates are generally used for long bones only, often for the aim being as temporary support.Plus most AM transmitters, aside from the USA, are rapidly disappearing, with FM and other delivery methods taking their place. Not many high power AM stations left, they are overwhelmed in built up areas with RF hash from millions of poorly shielded power supplies in use.

author: vinlove
author_rank: Frequent Contributor
timestamp: December 26, 2019, 06:18:50 pm
content: I saw someone using a tree as HF antenna, and he was making many good contacts with his portable QRP Chinese rig.

author: Bud
author_rank: Super Contributor
timestamp: December 26, 2019, 06:30:37 pm
content: What measures did he take to eliminate radiation from the coax? I bet he had no clue that the coax may radiate.

author: Nusa
author_rank: Super Contributor
timestamp: December 26, 2019, 09:03:51 pm
content: What good is a fire escape if you can't get to it? What if you had a fire in outside your door?

author: coppercone2
author_rank: Super Contributor
timestamp: December 26, 2019, 09:05:37 pm
content: Quote from: Nusa on December 26, 2019, 09:03:51 pmsupers like to break them to prevent break ins (usually in the hood). You might see a padlock on the ladder but its a stiff fine..

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2019, 07:28:40 am
content: so the problem with loop antennas at high frequencies according to manufacturers is that its no longer considered electrically short. Not sure what electrically short on an antenna is.100MHz is 3 meters.I assume the loop is considered a exposed wire or leaky feeder?. Not sure how you calculate electrical length on a ring, is it just circumference?If you go by circumference, your loop diameter would only be at most 10cm to keep it electrically short if you define electrically short as 1/10th.but, unless you are close, is that design even useful?https://www.semanticscholar.org/paper/A-VHF-active-receiving-loop-antenna-design-Ong-Tan/79bbbf00b2ecb90fff158a4986e0ec669ba85458

author: Nusa
author_rank: Super Contributor
timestamp: December 28, 2019, 07:37:21 am
content: Quote from: coppercone2 on December 26, 2019, 09:05:37 pmIrrelevant. As a tenant, he needs to be able to get to it from the other side, like his window, for it to serve its primary function.

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2019, 07:41:45 am
content: Quote from: Nusa on December 28, 2019, 07:37:21 amyea I thought like it might have 'doors' that are openable from the level you are on with a lever to access the lower level but don't allow you easy access to the higher level, but that falls apart if the apartment underneath is on fire and you need to get to the roof. But that its bootleg and you can open it with a stick or broom etc, so it might be chained up illegally. Also I think I had people knock on the window and ask for money when I was a little kid lolI think I have seen some kind of 'hatch' before on some buildings. Maybe it was just there to prevent people from falling down fire escape stairs if they go out there for whatever reason (like potted plants?)Not sure why you could not go on your own segment unless it rusted through or was dangling on the side of the building. Never investigated them because they were usually nasty and covered with bird shithas anyone seen a 'hatch' with hinges per level on these type? its bugging me nowhttps://imgs.6sqft.com/wp-content/uploads/2015/04/21001018/fire-escapes.jpgim thinking I must have been imagining it because a hatch might hit you on the head real hard. custom job from building owner maybe? im not saying a roof hatch, I mean a level hatch that looks like crossbars or even a lighter sheet metal doormaybe someone put little gates on something like this? (like the ones that prevent a dog from going into a room)https://p1.pxfuel.com/preview/693/189/119/fire-escape-stairs-apartment-dwell-royalty-free-thumbnail.jpgit all seems hazardous when there is smoke though

author: BrianHG
author_rank: Super Contributor
timestamp: December 28, 2019, 08:34:04 am
content: Do not laugh, I do not know why, but get a thick old all metal coat hangar, unwrap it and scratch off that semi translucent coating on the wire and hoot it to your 75 ohm antenna input lead. I have no bloody clue why, but it turns out to be the best cheap FM radio antenna around. (Not the thinner white painted coat hangars, the thicker yellowish ones)It's cheap and it works for really far stations. Note that it will not outdo a huge TV type directional beam antenna.