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Author Topic: yet another newbie hardware bunch of questions :-)  (Read 10920 times)

Posts: 4439

« Reply #15 on: September 28, 2012, 10:27:07 AM »

A 300 Ohm twinlead folded dipole similar to a common broadcast FM receiving antenna (except longer) is not a good choice for HF transmit. The conductors are too close together. A typical HF folded dipole has a spacing of at least a foot and often two in the radiating section. The lower the frequency the wider the spacing as a ballpark concept. You can find plenty of antenna ideas on the web and get handy with a search engine at the same time.

Since you have 50' between supports and we're talking rooftop, I agree with the previous comments about a simple dipole on 20 Meters. Very hard to screw one up as a DIY project. 33' tip to tip (16.5' each side of center) which allows 8' of separation from the supports or a little 'gimme' if you want the antenna feedpoint closer to one side than the other. The same idea on 40 Meters is 66' from tip to tip and not a good fit for your masts. Plus, the lower the frequency the higher the antenna should be for a decent take-off angle. Locate a 40 Meter dipole too low and the bulk of your signal goes up at a high angle which would likely give you regional coverage out to a few hundred miles. Hang a 20 Meter dipole in the same spot and your chances of working some DX are improved... Assuming daylight hours when the band is open.

One other idea you might want to consider is building a terminated folded dipole (TFD). Same concept as a folded dipole except there's a 300 to 400 Ohm (approx) non-inductive resistor opposite the feed point. Plenty of material on the web about them and you can build the antenna to fit your available space. Not as efficient as a dipole - but - they are broadband. So a 45' TFD with 24" spacing could load on 40, 20, 17, 15, 12 and 10 Meters without much complaint. You'll want a 4:1 current Balun at the feed point if you plan to use 50 or 75 Ohm coax and although 4:1 isn't textbook perfect, it has been my experience 6:1 and 8:1 Baluns tend to become lossy unless everything is perfect. Antennas often require some compromise so do not assume yours will be in the same Zip code as perfection. Design accordingly.

The TFD is a good general purpose RX / TX antenna and worth your consideration as it sounds like you'll be using one HF antenna for a while....................

Have you considered how many wireless devices are in simultaneous use near PyeongChang, South Korea?  The TV coverage alone requires thousands.  Then add in the cell phones.  Talk about a pileup......!     

Posts: 2462

« Reply #16 on: September 28, 2012, 07:30:21 PM »

Baluns designed for receivers typically use very small wire and the ferrite material does not have enough mass to dissipate the heat of a mismatch. A few things can happen;
The wire goes "zap" and no more balun
The ferrite gets really hot and permanently changes its characteristics, usually not for the better.

If you wanted to really explore things... heck, I would drop a few toroids in a mailer and send them to you. I have hundreds of them. I love making baluns, it is my version of knitting  Wink I could even send you a few balun design PDF's or some URL's to point you in the direction of rolling your own. You learn a great deal about those missing chunks of RF education that you would not realize just by memorizing the test pool for general class.

Home made wire antennas are fun! watch what happens when you start snipping away at wire, see where the SWR goes across the chunk of the band you would be authorized to use. Feel the "why" of how shortening an antenna moves the resonance upwards in frequency. Learn it like the early radio experimenters did. Nobody built gigantic aluminum monstrosities bigger than their house to sit 50' in the air.

Do not overlook the excellent opportunities of picking up a used rig at a hamfest. You will want to bring someone along to be your guide to help you check out things before you spend $250 on a used 100 watt multiband, multimode radio.

Amateur radio is like the time you bought your first car. It probably was not a Lexus or some high end vehicle. It was used, probably had a plastic bag dripping some unidentifiable fluid that was hanging out of the rust hole in the rear quarter panel and burnt or leaked as much motor oil as gasoline.

One of the big reasons on why people start up in amateur radio and then end up not renewing and are essentially out of the hobby within six months is frustration. Frustration with how noisy the bands are, how hard it is to start up or get into a QSO, nobody can hear you, etc... IMHO, while QRP is a fantastic aspect of the hobby it requires you to perfect all of those other parts of your station to get any sort of performance. You need the patience of Job, the hearing of a bat and sort of a perverse tendency to masochistic rewards.  Tongue

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
Free space loss (dB) = 32.4 + 20 × log10d + 20 × log10 f

Posts: 1757

« Reply #17 on: September 29, 2012, 09:38:28 AM »

You should take Tish up on her offer of a few ferrite rings and documentation.

Then for a few dollars you can build a high quality balun that will last for years and become a integral part of your kit.

73 de N4CR, Phil

Never believe an atom. They make up everything.

Posts: 6

« Reply #18 on: October 05, 2012, 09:32:42 AM »

Thanks again for all your ideas (and offer of toroids)!   I bought a used Icom 718 for $480 from some nice folks in Florida and ordered a Jetstream 4:1 voltage balun and PS from, and a General study guide from Amazon.  All should be arriving next week.  I'll put up a 10m dipole below the 20m and do some testing and transmitting on 10m while I'm still a Technician, and learn how to use the 718.

Speaking of learning, I'm learning new things every week about ham radio -- and that is one of the great things about it!  But as others have said, you can't learn it all at the beginning or buy all the greatest equipment right away --  you have to make a start with what ya got.  Making a rig or antenna work better -- and learning in the process -- seems to be a continual practice in ham radio.  However, as some of you have said, if it gets too frustrating, then you quit. 

I'll report back to this topic in a month or two, after I've made a few contacts!

Doug M
San Francisco

Doug McC.
San Francisco

Posts: 2806

« Reply #19 on: October 05, 2012, 03:05:17 PM »

With a rooftop 20m dipole, and an IC-718, you should be able to get lots of DX during daylight hours with PSK31 on 14.070.

And if you're willing to sacrifice speed, try JT65-HF instead of PSK31.  That will get you worldwide coverage at the right times of day.

As you say, the "hobby" is one of continuous learning.   That's what keeps it interesting.

Have fun --


PS -- electrical noise is the enemy of HF ham radio.  If your building is really noisy, you may be stymied in your efforts.   But you'll only find that out _after_ you get the rig and antenna working.

Posts: 1042

« Reply #20 on: October 06, 2012, 09:14:37 AM »

You will be a LOT happier with a 100 watt radio - you can always turn it down to 5 watts if you want to, but you cannot turn up a 5 watt radio to 100.

While you are at HRO in Oakland buying whatever parts you still need, pick up the ARRL Antenna Book or one of the other books on building your own antennas.  At the very worst you learn something, at best you find a perfect antenna for you QTH.

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