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Topics - gil

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361
Antennas / Building a 2m Slim Jim Antenna.
« on: May 09, 2012, 03:41:07 PM »
I needed a better antenna for my Yaesu FT-270R. My requirements were to find a portable, efficient and easy to build design. The short rubber antenna works fine for me now, especially that I am not transmitting before getting my license. I do want more range however for emergency situations, in case local repeaters are down. My first thought was to make a Yagi-Uda directional antenna. They have a high gain but transmit in only one direction. While this can be an advantage, and I plan on getting one, my go-to antenna needs to be omnidirectional. I found the Slim Jim design to be my best option. It is easily made from soldered copper tubing. All you need is a couple of 5ft lengths of 1/2" tubing, 90deg corners, end caps and PVC Ts. I added an electrical junction box for the feed-point connector, but it might make it more difficult to attach the coax and tune the antenna.

Total length very-top to very-bottom is 58". Width center-to-center is 2". Gap is about 1-3/4". See the Ham-Universe articles (1 & 2) for exact measurements.

It is easy to calculate dimensions for other frequencies:

  • 3/4w : 8415/F-mhz.
  • 1/2w : 5610/F-mhz.
  • 1/4w : 2805/F-mhz.
  • Feed point : 10-20% of 1/4w.

The gap is taken off the 1/4 wave element...

Slim Jim Gap

The PVC Ts need to be reamed with a 5/8" drill bit so that the copper tubing can go through. I used a bit of WD40 to slide them down. Between the two Ts, I epoxied a 1" piece of 1/2" plastic tubing. There is one "H" PVC support assembly on the top portion and one on the lower portion, right next to the end cap.

Slim Jim Feed Point

I am not sure that using a plastic electrical junction box was a good idea for the feed point. Since I have not received my SO-239 socket yet, I must hold off on the electrical connection. My concern is that soldering will be difficult without burning the plastic box. I might have to use sheet-metal screws. Tunig might not be easy either, since the best SWR is obtained by moving the feed point up and down, between 3 and 4" from the very-bottom. Maybe I should have used PVC Ts, like for the two support "H" assemblies. They can be split in half, then the coax soldered after finding the best feed point. Once epoxied, it would look fine. I do like the look of my electrical box though, and if it works fine, I will be happy with the results.

Total building time was about an hour. Everything came from the hardware store, except the SO-239 connector. Soldering turned out to be pretty easy. I sanded the parts and used flux paste before heating up the assembled parts with a torch. Once the parts are hot enough, you put the solder on, which flows in the joint, following the flux. Cost could have been as low as $30, but I spent about twice that much, not counting tools (hacksaw, drill bit, epoxy, solder), which you might have already. If you are starting "empty handed," plan on $100. While it can cost more than a factory-made antenna, you get the satisfaction of building something yourself, which might be actually sturdier than a store-bought model.

Stay tuned for the finishing touches (painting), electrical connection and reception testing.

Thanks to Richard KE5FXU SK at hamuniverse.com for the article!

Update, April 11th:

Finally, I got my SO-239 plug. Drilling the PVC box was easy. I didn't even use my drill press. It only took me a few minutes by hand! Holes are one 5/8" in the middle, and four 1/8" around. I drilled in the middle of the lid, hoping I would have enough leeway to adjust the SWR by moving the contact in the box along the tube. First, as I suspected, I could not get the tube hot enough to solder the center of the coax to the copper tube using my 30w soldering iron. I solved the problem by heating up the tube with a Zippo under it while I soldered on top! It worked really well. I did the same to put soldering points in the box, every quarter inch or so. The zippo was placed an inch from the box. I was worried about melting it, but these electrical boxes are pretty heat-resistant. Sorry about picture quality:

Solder Points

It's a bit ugly, but inside the box anyway..

Feed Box

Reception works great. I was able to listen to a conversation tonight on a distant repeater that I simply could not hear with the HT rubber antenna. I get three extra signal bars with the Slim Jim. I got a cheap VHF/UHF digital SWR meter from Hong-Kong, which seems to work fine, but for the connectors which are of "N" type.

Digital SWR meter

Update, May 1st:

I painted the antenna sort of a flat olive-drab color for stealth. I can easily hoist it up a tree and it blends in very well. SWR varies from 2.4:1 on the lower part of the band, to 1.8:1 around 146Mhz, and remains around 1.4:1-1.5:1 from 146.5 up. I used a ferrite RF choke kit from Palomar Engineers (photo below), which got the SWR down to 1.36:1 around 147.5Mhz. I much prefer the ferrite choke to the coax balun type, which looks ugly and wastes cable.

Slim Jim Connection Box With Palomar Ferrite Choke
 
What I like the most about the Slim Jim is it's sturdiness and that given it's shape, you can hang it from anywhere, as long as you use an isolator to do so. When hanging it, I plug in an "L" shaped adaptor (photo above) to avoid bending the coax.
Update Oct 30, 2012: I have been using the antenna with my Icom IC-271A and it works great, even with the antenna set on the corner of my desk! I do plan on putting it outside on a pole or hoisted in a tree, but haven't had the time. I can hit distant repeaters on half-power (13W) with no problem and get good reports. What I like most about it is that I can take it with me anywhere and not worry about banging it or damaging it. It is easy to hoist up with a string or tape it to a pole. The SWR can probably be reduced by doing a better job at finding the sweet spot to attach the coax. It doesn't even look like an antenna to the untrained eye, which is always a plus.

362
General Discussion / Frequency considerations and introduction.
« on: May 09, 2012, 03:09:47 PM »
I bought my first CB radio in 1984, a Lafayette LM300. I wish I still had it. Ten years later I got a President Jackson. After CB went ?down the drain,? I stopped all radio activity, until today.

I clearly remember twenty years ago, even five. I did not then have any of the concerns I have today. The future was bright with no clouds on the horizon. I don?t think the world is going to end this December twenty first, or next year for that matter. The Mayan who wrote his calendar must have been tired that night, and his wife was complaining about how much time he spent on it, so he probably just thought it went far enough and left it at that.. I am no doomsday preacher.
There is a certain unease among us however. The media is broadcasting multiple disaster and prepping shows. The economy isn?t going better. Five years ago, I had five ounces of gold, bought for less than $1500. Today, those coins would be worth close to $10,000. Only five years later! I am still hitting myself on the head for selling them before the increase. We may wonder why the value of the Dollar and the Euo have not gone down by a factor of six or seven. One only has to watch the news to start worrying.

I have always been the poster child for safety. Yet, I have enjoyed dangerous activities like ultralight flying, cave diving, motorcycle riding, parachute jumping and others, but always prepared and trained properly. The fact that I am here today has nothing to do with chance.

Being prepared for me means being prepared in all areas. I have seen people stocking up food while smoking two packs a day, shooters with dozens of guns who can't run to their 100yrd target, or people with a bunch of medical supplies but no water or any means to purify it. I have seen very few preppers with the means to communicate!

So, I am studying for my HAM licenses (yes, all of them), while enjoying my Galaxy DX 979 CB.

I have thought about HAM bands quite extensively and done a lot of research with an emphasis on emergency use.

For local communications, 2m seems the best option. I would also include a few FRS handhelds to keep track of family members and local friends in your neighborhood. I can't think of anything the 70cm band offers that 2m can't do.

A CB is a must as well, but it should include SSB, to take advantage of the 11m band's ability to skip on the ionosphere, thus allowing very long distances.

I am also intrigued by the 6m band, which at times behaves either like VHF or HF, depending on conditions. It also has the advantage of small antennas, with a half-wave being, well, 3m long! I found an old 6m Gonset Communicator III AM radio on Ebay for cheap, with a crystal for 50.4mhz. That set would probably survive an EMP sitting right in the open! If only someone else around had one...! 6m AM would make a great local frequency, if small and cheap handhelds were available. You would also get skip when the conditions are exceptionally good.

On HF, I am planning on sticking to 20m (14mhz). Lower than that and you run into antenna length problems. You can find cheap QRP kits for that band. I have ordered a small CW only transceiver kit for $30! How can you beat that? 20M is probably the most popular HAM band and would be great to listen to in a national emergency. Moreover, propagation is decent to great most of the time during the day and early evening hours.

In my opinion, better have a few radios for different bands rather than one that covers them all. I do want a Yaesu FT-817ND, but I will have a metal can with a 2m handheld, CB, 6m SSB, 20m QRP transceiver, a handful of FRS handhelds and a few accessories stored inside a cardboard box inside the can, for EMP and water protection. For a few hundred dollars, you can be well prepared as far as radios go. Much less than the cost of a good rifle ;-)

363
General Discussion / PLEASE READ! Welcome to Radio Preppers.
« on: May 09, 2012, 02:11:40 PM »
Information is a vital commodity in a disaster situation. Whomever has any, in the absence of Internet, phone and power services will have a great advantage.

Radio Preppers aims to provide individuals interested in disaster preparedness with an independent tool for the exchange of information about emergency radio communications and preparations. Hopefully it will also help build a community of like-minded individuals who could contact and help each others in times of what is commonly known as 'SHTF' or 'TEOTWAWKI.' Like-minded here means self-sufficient, strong-willed and responsible people. Independent means regardless of nationality, race, gender, political and religious beliefs, as well as unrelated to any organizations. Whether you are a licensed HAM operator, CBer, or simply curious about radio preparedness doesn't matter here.

My motivation for creating this site came from my inability to find an emergency radio club that really wasn't related to some kind of organization, mostly governmental or politically affiliated. Survival is a personal, family or small community affair. I am always suspicious of organizations that plan on telling people what to do for their own good, or else... That said, anti-government rhetoric will not be accepted here. If you don't like your government, vote accordingly. There are plenty of other boads for political ranting. This one is not one of them.

Sign-up, it's free, and stop by once in a while. If you have anything to contribute, please do so! Topics will not necessarily be limited to radio but must be related to disaster preparedness. To avoid spamming, you do need to answer a couple radio related questions; nothing a quick Google search can't answer.

Rules are few: Be courteous. Although some civil political discussions are acceptable, try to avoid them; same goes for religion. Do not suggest anything against the law, or you will be immediately banned.

Please consider supporting this site after joining by subscribing at: Profile > Summary > Actions > Paid Subscriptions. You will get more privileges!

Note that members who do not participate at least once a year will be deleted.

Any suggestions would be appreciated. I hope you enjoy this forum and that it helps make you and your family safer.

Gil.

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