Sunday, March 31, 2013

Salt Water and Longwire Antennas

My family had the opportunity to visit our oldest son down at Embry-Riddle in Daytona Beach this past week while the kids here were out for Spring Break.  Although it was not the typical Florida weather with temps dipping into the 40's -- we managed to have a great time.  Fishing was fun, and Kennedy Space Center is just an amazing place.   I did manage to pack up the '706, some wire and an antenna tuner just in case.... and glad that I did!  Learned a few things in the process and had some fun when the weather was keeping us indoors.

Along with a manual antenna tuner, I opted for a longwire antenna (spool of stealthy black 22 gauge stranded hookup wire) since it was unknown what kind of access and arrangements would be allowed at the hotel.  As it turns out, we ended up in an end unit on the top floor (6th) -- about 55' or so above the parking lot facing the Atlantic.  

The building curves around north to east and another condo complex next door pretty much blocked views to the south; but that just increased the challenge a bit to get a decent signal launched.  The salt water 250 yards distant certainly can help, right?

The spool of Radio Shack hook up wire (maybe $7 ?!) was 95' feet long, but I quickly found it presented some high reactance on several bands at that length -- a bit more than the MFJ Versa Tuner II liked, so I trimmed 6' off of it that first night, and that seemed to do the trick on most of the bands to keep the tuner within its range.   The wire ran directly from the upper rail of our balcony down at about a 40 degree angle to a fence that went toward the beach, east-southeast direction; installing this at dusk several evenings prompted a few funny looks, but it was essentially invisible and well above any passerby's heads.  Using a couple "figure 9" carabiners on each end of the wire and some braided nylon line to tie off made the job a simple under 10 minute process each evening.

A loop in the wire up near the balcony hooked to the carabiner, allowed the slack "feedline" to make its way to the tuner.  A short 4' piece of RG8X with alligator clips was used to connect the longwire to the tuner and to ground the braid near the feedpoint on the metal balcony.  (I could have used the balanced high impedance input and some ladder line, but didn't bring any on the trip...so I was stuck with the 50 ohm coax connection...probably would have loaded better with that approach.)


I loaded it up on 20m the first evening, and although the tuner was a little touchy finding the right capacitor combinations -- a 1.2:1 match was obtained in fairly short order.  I answered a couple CQ's using 100w and was receiving very mediocre reports;  for example a loud W5 running 100w and solid 59 at my QTH, gave me a "weak" signal report of 53, just above his local noise.  Other QSO's were similar.  Why the report mismatch? What about the theory of antenna TX/RX reciprocity? Antenna Reciprocity  Well -- its is still there of course -- all day long -- but power losses on the transmit side in the form of lossy components and/or ground losses don't really impact the S/N ratio on the receive side the same way (try to transmit with a Beverage for example) .......soooooooo...... Where is the transmit power going?   It is either going to radiate or heat something up.  Pretty sure I was heating something up the hard way (tuner, coax, chassis, etc...).

As it turns out, the RF Ground (aka counterpoise) for the longwire and other unbalanced antennas is crucial, despite me and generations of other radio amateurs wishing otherwise.  A poor ground means that the return current faces real resistance (Rg) which ends up consuming,  and not radiating, power -- the better the ground plane / counterpoise, the less loss and the stronger transmit field strength. (A good description on the importance of minimizing Rg  is here)   I went back to work and made a solid ground connection from the braid at the feedpoint to the all aluminum balcony and the doorframe -- and for good measure connected about 150' of excess wire and twin lead laying on the balcony floor to the same ground.  Readjusted the tuner to the new impedance, loaded the antenna, and boom -- excellent signal reports all around on 20m.  More importantly, the reports implied that the antenna efficiency had improved and a 59 that I handed out was returned with a 59, a 55 with a 55 and so on.  (It is instructive to again witness that SWR is never a complete measure of effective antenna performance, only how happy your transmitter will be; after all, a dummy load presents a 1:1 SWR -- but is about as inefficent a radiator as you'll find.)
W4QA/4 on 6th Floor - Daytona Beach 250 yds away

After these changes, results were above my expectations on most bands, with 40m and 30m being the most productive when I was QRV as W4QA/4 -- and no doubt the salt water proximity didn't hurt.    Several stations on 30m remarked how loud the signal was and wanted the details of the antenna -- with more than one asking "only 100 watts?".  On these two bands I managed to log about a hundred contacts into maybe 30 countries with some casual operating in the evenings and early morning hours.  More than anything it was good fun, and I "relearned" my counterpoise lesson from my Novice days after getting more than one RF burn on my HW-101 due to RF in the shack from lack of a good ground.   It's also motivated me to learn more about Maxwell's concept of displacement current (i.e. the "imaginary current" running through the air between poles of an antenna, or the radiating element and ground) and why, after about 140 years, there is still a debate on what this actually is -- but that's a whole other story.....

The longwire hasn't been held up in amateur circles as a great antenna for a number of reasons, and I suspect much of its reputation has been due to the kinds of ground problems experienced here.  Reports of RF in the shack, poor efficiency, combined with unpredictable (well, without software at least) radiation patterns probably pushes away a lot of folks before they actually try it.  ("Hey, I've got a great SWR, can hear all kinds of stations, but just can't 'get out' ")    Also, antenna manufacturers don't exactly see the profit in selling you a spool of random length wire --- now tuner manufacturers, that's another story.  Focus on providing a decent ground and putting the wire up a significant height above ground however, and you've got something that can perform well on multiple bands (with gain in certain directions (!) ), costs very little, and is simple to install --- ideal for portable operations or limited space / budget QTHs.   I'll be doing it again next time and may just have to put one up at the home shack for some more experiments.


73,
Rush, W4QA

1 comment:

  1. Hi Rush. I have a couple of questions about your antenna setup, specifically having to do with grounding. If you see this, give me a quick call on my cell 703-675-3144. It's easier to ask my question with a quick conversation than to type it here. I am also a Hokie (Class of 1990). Thanks, John.

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