Ideas to make the smallest 160m antenna ever. - Page 1
Forum section: electronics / rf-microwave

author: Bhutt
author_rank: Contributor
timestamp: April 20, 2020, 10:16:44 pm
content: Looking for ideas to make the absolute smallest 160m antenna. Just need to transmit CW over a few KM's with a pair of experimental Michigan Mighty Mites.Is it possible?

author: TheMG
author_rank: Frequent Contributor
timestamp: April 21, 2020, 12:36:40 am
content: A mag loop antenna would likely be the smallest practical antenna, it does have the disadvantage of narrow bandwidth, requiring tuning of the variable capacitor (this can be made remote-tunable with a gear motor) if the frequency is moved more than a few kHz. But they can be extremely efficient for their small size and don't require a ground plane.

author: radiolistener
author_rank: Super Contributor
timestamp: April 21, 2020, 12:58:08 pm
content: Small antenna is possible, but any antenna requires about half-wavelength empty space around it. For small antenna it is even more critical. Because small antenna uses extreme high field strength and any object which falls into near field antenna region will affect it's performance more dramatically. This is why largest antenna works better, because it is less sensitive for nearby objects.Another issue with small antenna is very narrow bandwidth. This is because small antenna requires high Q-factor. It means that you will need to tune it more often.Third issue is that small antenna uses extreme high current and voltage. It means that you will needs to use low resistance materials and high Q and high voltage capacitors in order to reduce heat loss.Example of small antenna is magnetic loop, as mentioned above.

author: Bhutt
author_rank: Contributor
timestamp: April 21, 2020, 01:30:56 pm
content: Good morning,Thank you for the advice, so an actual antenna is out because even the magnetic loop would be too big. I was hoping for some way to make a rubber duck size one.What I am trying to do is to be able to have a shortwave receiver pick up a signal generated on one specific frequency. My son spends every other week at his moms place which is according to Google maps Measure function 417.5 meters or 1369 feet away on the other side of the river.We both have shortwave receivers and we are both going to make a modified mighty mite from two plans I have combined from the MAKERF.com website.I have attached a picture. The crystals on the way are 1.8432 mHz.What about if I extended the connection (drawn in red) to a few inches long before the dummy load? Would that emit enough signal to travel the distance reliably?Any suggestions for modifications would be great!cheers,

author: OwO
author_rank: Super Contributor
timestamp: April 21, 2020, 02:49:18 pm
content: That distance is better covered by VHF (FM radio). At 1MHz 400 meters is just over one wavelength away and is considered inside the "near field". A non-radiating near field falls off exponentially with distance, so you would need a very strong field at the transmitter (a tesla coil will do for sure).

author: CaptDon
author_rank: Super Contributor
timestamp: April 21, 2020, 04:11:43 pm
content: There is no substitute for 'metal in the air'. You may try some sort of 'long wire' even if thelength is only ten feet. You have one bad thing working against you (besides a poor antenna)and that is man made noise at that frequency. The noise floor is horrible at that frequency andyou will soon find every switched mode power supply device in your house transmits betterthan you do. Why so small?? So the kid can hide it??? C'mon!! The river works in your favorwith good local ground conductivity. The antenna could be a piece of wire hanging out thewindow and may work if you are in visual range of each other. Why not just buy a set of FMRSwalkie talkies like hunters and fisherman use??

author: radiolistener
author_rank: Super Contributor
timestamp: April 21, 2020, 05:19:57 pm
content: Quote from: Bhutt on April 21, 2020, 01:30:56 pmIn such way you will get traveling wave antenna. But you will need a very long wire in order to get acceptable performance on such frequency. It's main lobe will be directed along the wire from transmitter to the dummy load.In your case the best way is to use just a long wire with antenna tuner. For example, you can build it from self made coil and air capacitor from old tube receiver:For 160 meter and variable capacitor 10/430 pF, you will need inductor for about 35 uH. You can tune antenna with variable capacitor by listening for a maximum signal level in receiver.In order to calculate inductor, you can use some online calculator for air inductor, for example:https://m0ukd.com/calculators/air-cored-inductor-calculator/Use inductor diameter equals to coil length for better Q-factor. For example 26 turns on 75 mm diameter and 75 mm coil length. If it will be more easier for you, you can use smaller diameter, for example 59 turns on 30 mm diameter and 75 mm coil length, but it will give you a little worse Q-factor. Use uniform winding with the same distance between turns - this is important

author: 9aplus
author_rank: Regular Contributor
timestamp: April 21, 2020, 06:39:50 pm
content: Consider to go up on 24, 27 or 28 MHzhttps://sa555.blogspot.com/2011/07/24906-mhz-qrp-tx.htmlThan piece of wire 1/4 wave is small enough to hang from window.Or you have ready PC TCXO second hand modules on 27 - 28 MHz than just add keyed buffer and PA.

author: Bhutt
author_rank: Contributor
timestamp: April 21, 2020, 06:42:56 pm
content: Thank you for the replies,What I am trying to do is to be able to have two decent shortwave receivers pick up a signal generated on 1.8432 mHz from less than 1500 feet away.We both have shortwavereceivers, my son should have a long wire for his shortwavereceiverup by the time we complete the builds of the simple CW transmitter shown. The transmitters do not have receive capability so I do not understand how noise from appliances would interfere, does this noise get injected into the CW transmission? I do not have a problem with noise when I am SWLing down in those frequencies at this point.My goal is to have a fun way for us to learn morse code only, that's all, and would not be a mode of conversation beyond this.The attached picture of the transmitter I believe may put out 1/2 watt. If it wasn't for the few houses in the way it would be line of site and just a few blocks away as the crows fly. I would almost guess at the height of my long wire antenna for my receiver, it would almost be line of site with the exception of a few chimneys....From what I understand by the comments is the signal direction travels along the wire to the dummy load, could the shape of the wire direct this tiny signal to our benefit?,Cheers,

author: Marco
author_rank: Super Contributor
timestamp: April 21, 2020, 07:01:15 pm
content: Quote from: radiolistener on April 21, 2020, 12:58:08 pmOr use class-D ... you're HAMs, I'm sure you'll take care of the out of band emissions.

author: Bhutt
author_rank: Contributor
timestamp: April 21, 2020, 08:01:23 pm
content: Yes I am a licensed operator for all amateur bands, Also you may have notice thelow passfilter I added in the original picture of this 1/2 watt experimental radio that should address the second concern from the standard Michigan Mighty Might. Saying that, having to explain myself to someone I don't even know, is not why I joined this forum or to be policed in ayou better have...manner, but rather to ask for some guidance, ask could this work type questions and to have a little fun while doing it.Cheers,

author: dropkick
author_rank: Contributor
timestamp: April 21, 2020, 08:43:22 pm
content: I'll agree with the long wire post. At this distance if it radiates at all it will probably work, save for noise issues and don't worry about SWR or length. Get the wires broadside to the other end rather than pointed at it and in the same horizontal/vertical plane. Could also try an end-fed halfwave if you want to get fancy.

author: TheMG
author_rank: Frequent Contributor
timestamp: April 22, 2020, 12:34:14 am
content: Since you're operating on a single fixed frequency and in CW, bandwidth of the antenna or the need to re-tune is not going to be an issue, and due to the short distance the poor antenna efficiency should be manageable, so I would try the suggestion of a random wire with a simple tuning circuit. Cheap and easy and if it doesn't work out you won't have wasted too much time and materials.

author: Bhutt
author_rank: Contributor
timestamp: April 22, 2020, 10:26:28 am
content: Thank you folks for the input, very much appreciated. When we get a chance to build the mighty mites will let you know how things turned out.Cheers,

author: radiolistener
author_rank: Super Contributor
timestamp: April 23, 2020, 05:31:10 am
content: Just use long wire with antenna tuner fromthis post.Such tuner works very good for power about 5W and below. For higher power electrical breakdown occurs in variable capacitor due to extreme high voltage.Place variable capacitor and inductor as close as possible to antenna wire. You can use 50-ohm coax cable to connect tuner with transmitter.Antenna tuner is needed in order to get acceptable antenna efficiency. With no tune, your long wire efficiency will be very low and communication distance will be very low.

author: arno
author_rank: Contributor
timestamp: April 23, 2020, 10:27:46 am
content: Hello,+1 for the end-fed long wire. Using one with ~10m in my flat, tuned with variable L and variable C. Although you are licensed, please be careful, I am transmitting only 5 Watts and can reach whole Europe at night using FT8.73 de Arno, DL2SSB

author: tkamiya
author_rank: Super Contributor
timestamp: April 23, 2020, 05:52:27 pm
content: I've done these long time ago.1) Light bulb on top of a mast fed by coax2) Random length of wire attached to center pin of a dummy load3) Random length of wire attached to L network (antenna tuner)4) Just a random length of wireUsing 10 watts, I was able to have QSO with someone 5 to 10 miles away.

author: frogg
author_rank: Regular Contributor
timestamp: April 24, 2020, 10:45:11 pm
content: I suppose you can look at it this way:At 160m, Chu considers an antenna that fits within a sphere of radius 25m is considered "electrically small". So I guess if you're going to go small, why not go really small?Let's say that you want to make a very small 160m antenna, let's say 10cm radius.The Q is calculated with a wavenumber of 0.039 radians/m. The third order term dominates the calculation, leading to a Q of about 1.65*10^7.at a frequency of 1.87MHz, this means a bandwidth of .11hz.Maybe someone else can check my math, but given the above, and a suitable matching network, then it should be pretty simple to figure out how much ERP you're getting out of your system (your CW power, CW signal bandwidth, etc.)

author: fourfathom
author_rank: Super Contributor
timestamp: April 24, 2020, 11:40:55 pm
content: Quote from: frogg on April 24, 2020, 10:45:11 pmThat may be the Q you get with a zero-loss antenna (perfect conductor, zero ground losses, etc), but it's definitely not going to be the case with any practical antenna, ground, and matching network. For very short antennas the antenna impedance will be largely capacitive, with an extremely low radiation resistance (tenths of an Ohm or less). You typically use a series inductor and shunt capacitor to transform this impedance into something that you can drive with a signal source. A good series inductor may have a Q of 100 or so, and this will dominate the bandwidth and loss numbers. In the above case the loss will be very high. This may be a good thing, since the calculated bandwidth of 0.11 Hz would make it difficult to pass any audio-modulated or usable digital signal.But other good news is that the receiver performance will be dominated by atmospheric and man-made noise, so loss in the receive antenna can be high. I suggest that the OP try the resistively-loaded coat-hanger antenna or something similar and see how far they can make it work.

author: fmkit
author_rank: Contributor
timestamp: April 25, 2020, 12:13:42 pm
content: I'd use Qi charger, easily tuned after removing 200nF caps. Norma load is 5-15w and original shield makes the antenna directional.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: April 29, 2020, 07:46:43 am
content: Thought it's not efficient a 1.5m diameter magnetic loop is capable of doing more than what you need even on 160m.http://home.alphalink.com.au/~parkerp/projects/projmag.htmI've had SSB contacts up to 300km with it when used indoors. And with 10w AM more than 10km (video demo below)Even if you go for a short length of wire impedance transformation is highly desirable and this is more difficult with a shorter wire.