HF Antenna Construction

einherjar · 08-25-2012, 18:27

When building an HF radio antenna for a specific frequency do I have to cut the wire or can I simply wrap the excess?

I have a SOF-230 HF Antenna Kit from C&S Antennas. The kit includes (4) Antenna Wires each 63' in length. The wires are insulated and terminated with very well constructed connectors and strain relief, obviously not meant to be cut.

The documentation provides a frequency to antenna length table and vaguely states "see Table 1 and use the given length for the antenna wire."

My hunch is that simply wrapping an insulated wire does nothing to determine its electrical length and will make for a pretty poor antenna.

C&S Antennas

SOF-230 Antenna Kit Brochure

**Peregrino** · 08-25-2012, 21:55

It's obviously defective; I'll be happy to take it off your hands.

IIRC antenna matching units and terminating resistors make up for a lot of frequency/impedance mismatch. Normally, I would say you're correct WRT the insulation; however, that's an expensive antenna with a lot of science involved in its design so I suspect we're both missing something in the equation. The brochures are worthless, do you have the manual? (See if T7L remembers any of his HF commo training.)

When I went to O5BS in '79 we built our antennas from W-1 (procured by an enterprising instructor at no cost to SWCS) by stripping the insulation from the radiating element starting at the highest frequency (shortest electrical length) out to the terminating end. That way, when we needed to change freq it was a simple matter of rolling/unrolling to the requisite length. The stripped radiator ensured the coil shorted to the correct frequency. I remember spending part of my meager paycheck at the Radio Shack in Eutaw Village buying antenna components before the final FTX. Blasted instructors wouldn't let us use the antenna kits that came with the radios; something about "learn by doing" and "if you fumble fingered idiots screw up the issued antenna, SWCS cannot afford to replace it and it has to be complete/correct for the EoC inventory". Lord forbid we're ever in those straits again.

SF_BHT · 08-26-2012, 02:25

If you are going to be using a set freq then we used to make a custom antenna. But normally due to the daily freq change we just rolled out or in the proper length and made commo. Cutting your antenna to freq is a term not a literal requirement.

Good luck

2018commo · 08-26-2012, 05:04

Quote:

Originally Posted by Peregrino

It's obviously defective; I'll be happy to take it off your hands.

IIRC antenna matching units and terminating resistors make up for a lot of frequency/impedance mismatch. Normally, I would say you're correct WRT the insulation; however, that's an expensive antenna with a lot of science involved in its design so I suspect we're both missing something in the equation. The brochures are worthless, do you have the manual? (See if T7L remembers any of his HF commo training.)

When I went to O5BS in '79 we built our antennas from W-1 (procured by an enterprising instructor at no cost to SWCS) by stripping the insulation from the radiating element starting at the highest frequency (shortest electrical length) out to the terminating end. That way, when we needed to change freq it was a simple matter of rolling/unrolling to the requisite length. The stripped radiator ensured the coil shorted to the correct frequency. I remember spending part of my meager paycheck at the Radio Shack in Eutaw Village buying antenna components before the final FTX. Blasted instructors wouldn't let us use the antenna kits that came with the radios; something about "learn by doing" and "if you fumble fingered idiots screw up the issued antenna, SWCS cannot afford to replace it and it has to be complete/correct for the EoC inventory". Lord forbid we're ever in those straits again.

I went through in the spring of 1980 After building one out of hard drawn copper and woumd on reel to reel spools, our TAC showed us how to use a grid drip meter to find resonant points. The tallent on the Phase II staff was amazing. To the OP, depends on your usage (fixed/ mobile) and transmitter limitations.

glebo · 08-26-2012, 05:04

As SF BHT stated.."cutting" is just a term. You just measure the calculated length and roll-unroll how much wire is needed.

Also, a "rule of thumb" is to take a foot off each end of a resonant antenna (one cut to frequency) to compensate for "end effect", it helps reduce feed point impedence.

That kit is pretty good, I saw one the other day out at the commo committee, they are in dire need of new antenna kits, because the old SORAK kits have seen their day.

The kit comes with two BALUNS, one for resonant antennas (ratio suites 1/2 wave feed point impedences, I believe it may be a 1:1.5 or 1:2 ratio) and one for non-resonant (terminated long wires/sloping "V"'s etc), I believe that one is in the neighborhood of 9:1 to match feed point impedences of long wire antennas.

Just make sure you use the right BALUN for the system you are construcing, IE resonant (cut to freq) or non-resonant (multi-freq terminated with a resistor) and you'll be good to go.

Good luck..

ETA, I was just at Campbell for 4 days to visit my son, I didn't have internet or I would've looked you up...ooops.

albeham · 08-27-2012, 05:56

This antenna system is to be used with all of the wire out and not wrapped at all.

If you wrapped the excess wire back on the roll, it will make a nice coil, reactance will cause the antenna not to work as designed . yes the antenna tuner will tune it, but the radiation pattern will be effected. This effect will affect the antennas patterns making the predicable pattern to be a guessing game.

Only bare wire can be wrapped back on the roll, this will be a good electrical connection. With insulation, it will not happen.

The antenna system is designed to be used over a lot of frequencies. If you want to "cut" a antenna to one frequency use a dipole and bare wire. 4:1 balun,

any question hit me up..

AL

T7L · 08-27-2012, 10:39

einherjar,

albeham is correct. Your antenna is not one you would 'cut'.

If the antenna is using insulated wire the electrical length of the annenna will not change if you roll the wire on itself. It will create irregularities with the radiation pattern of the antenna. Peregrino's description of stripping the wire at predetermined lengths for their antenna allowed him to use partially insulated wire for a 'cut-able' antenna.

If you are trying to make a resonant length antenna that you can change the length as you change frequency, you are better off using un-insualted wire and a 4:1 balun as albeham also stated previously. This is especially useful for RTs that dont feature a built in antenna matching unit. (I acquired a FT-817ND this summer. The 817 doesn't feature a built-in tuner or matching unit, however my suprlus green HF radio does)

ARRL.org has some very good resources for antennas and construction. Their antenna handbook is excellent. You can also find detailed instructions for balun construction from various ham websites if you need something specific and dont have time to order it. Two examples - http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=16753 and http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=21420 There was a good bit of scrounging and fabrication in the '90s. I too, hope we aren't headed back to those times.

If you can find an old GRA-50 antenna kit, PRC-70 kit bag or SORAK (OE-452) it has rather good wire for variable length antennas.

Hope this helps, and hit me up if need anything also - T7L

Badger52 · 08-27-2012, 13:52

Quote:

Originally Posted by T7L

(I acquired a FT-817ND this summer. The 817 doesn't feature a built-in tuner or matching unit, however my suprlus green HF radio does)

I hope you enjoy your 817 as much as I have. Given the low output its tuning companion is a little Elecraft T1. Both take up less space in a bag than "all the other stuff."

[/hijack]

albeham · 08-28-2012, 05:23

Looks like we all have a heart for the low power stuff.

I too run a FT-817 with an end feed 1/2 wave, "cut" to freq. It kicks butt. Take it on all the hikes, can't get away from spending time in the field. Gets me out real good, no real weight. We might need to figure out a god CW round table.
AL

T7L · 08-28-2012, 05:28

Badger52,

The 817 is nice, especially when your low power HF experience was gained on the PRC-70, 104, MOD 50, etc. Tons of features in such a small package. It's nice to have your radio, two reels of wire, lanyards, weights, balun, etc, etc, fit in a 5.11 PUSH pack with plenty of room to spare.

I was looking at either the T1 or Z-817 as tuner options. Definitely will be going with the T1. Elecraft makes good stuff for Hams. An old boss of mine just finished putting a KX-3 together. Thanks for the advice. - T7L

Badger52 · 08-28-2012, 05:51

Quote:

Originally Posted by T7L

I was looking at either the T1 or Z-817 as tuner options. Definitely will be going with the T1. Elecraft makes good stuff for Hams.

Roger that, think you'll be happy. If one is so inclined it can be done as a kit or they make it ready to go. That thing'll find a match on a coathanger.

Back to the OP, it seems the intent of this SOF-230 setup is not that dissimilar from the AS-1743 and pretty much leads folks operating very short-term to leave wire wrapped up. What's the extent of the compromise to the pattern, if you know, between leaving it around its keeper and just letting it all out? Not having any specs on the tuner aspect of what's being used can't really tell how much latitude that matching unit (of whatever kind) has.

69harley · 08-28-2012, 07:39

Amazing to see such a small, rechargable HF system. I bet it wouldn't be too hard to rig up a cable to interface it with an old DMDG. I would have traded an organ back in the day for some something this small and lightweight. Does anyone know how long the batteries last on a full charge with this little radio. I can't beleive I am thinking of buying a backpackable HF rig.

Badger52 · 08-28-2012, 11:51

Quote:

Originally Posted by 69harley

Does anyone know how long the batteries last on a full charge with this little radio.

Wow, this thread certainly seems to have morph'd. The answer is "it depends" upon the duty cycle you impose on the radio. The batt pak comes full at about 11V and the spare Alkaline pack gives you about 10.2 maybe. On a decent gel-cell several actual operating hours with a reasonable duty-cycle, xmit/recv. (I use a 5AH model simply because that size works for my bag & most circumstances, tote what you wanna tote.)

There are some other things you can tweak on the radio to lower its basic consumption quite a bit. The biggest one is this:

If you do NOT also need the SO-239 on the back, run all your HF out the BNC on the front, this is selectable in the radio menus. Just having that SO-239 enabled is a current draw - wtf, NFI. The only thing I'd further recommend is a short patch cable to an HF adapter so as not to strain the BNC right on the front panel. Think everything lightweight and whatever makes it easy on the miniature physical interface on the radio. The other big one to save battery is use an ear bud or phones rather than spending your battery driving a speaker.

BTW, the duck, as it comes with the radio, is pretty damn good. If it can see its destination within reason, it'll get there. I hit the local repeater 16 miles away on the duck, in the upstairs bedroom, and from inside a lunatic-driven, storm-spotting car as well.

But it's nice to be able to run both interfaces if needed. Once in awhile someone actually staffs the HF rig in the nearby NWS office...

If SSB is your thing, get a filter maybe, this isn't some Yaesu console rig. I did flip the money for an older Collins CW filter (chip slot inside the rig, simple menu change to recognize it) and very glad I did. Doesn't attenuate like alot of the modern digital stuff.

Just remember it's 5 watts (mine's typically 4.8); I personally think it's a better little VHF and CW rig because of SSB's demands. Don't get me wrong, I do have a regular base station, and a whole QRO setup (amps are for friends) and I always say the first 400w are to leave the double-canopy suburban RF-hell on my block. But if you're willing to pack a lunch and throw some wire up & get it in the clear it's a pretty versatile choice especially on CW. Antenna, antenna, antenna. When it's workin' it's a blast. I've heard from a couple folks that Elecraft's little rig has an awesome receiver section in it btw. Jes' sayin'.

As to backpackable HF, we've all seen some pics of what someone else told a vintage QP was "packable" - this would be, um, better. Naturally, to save batt draw, there's nothing wrong with a few of the smaller 100w radios and simply turning them down. Some pics of what the 817 excels at and then back to watching 'the thread.

**The Reaper** · 08-28-2012, 17:37

I was just window shopping ham rigs today and saw the 817/857.

Tiny package, looks very nice and portable, if you can afford it.

$700 for a radio (plus several hundred more for accessories) is a little rich for my blood.

TR

Badger52 · 08-28-2012, 18:49

Quote:

Originally Posted by The Reaper

$700 for a radio (plus several hundred more for accessories) is a little rich for my blood.

TR

Roger that sir, their pricing kinda went nuts after a bit. The current one is the 817ND. If you don't care about the few freqs available on 60m you can often find a good ol' 817 for much less. Quite a few folks get disillusioned with low power and then let it go. Still, seems anything brand new these days they're pretty proud of at the cash register.