RSS

Thoughts on Antennas-Two Years Later

25 Nov

Inasmuch as Blogs are like a diary of sorts, I thought I would revisit my primary considerations for properly designing and erecting a HF antenna on a city lot. This piece is written in the context of two years of experimenting with a smallish dipole on a suburban lot.

For most of us, large towers and big arrays high in the sky are not something we have access to. This is true for me and most suburban Hams I know. Yet, with all of our limitations we all seem to want to work DX, enter casually a contest or two each year to flesh out our country totals or just enjoy reliable use of HF communications with no tolerance for creating havoc within our own or our neighbors electrical appliances. With those caveat’s in mind I review, yet again, what I believe to be important when considering an antenna installation. This list changes with things I have learned. I expect it to change again.

Assuming you may think some of this germane to an installation you are considering-enjoy. It is understood that you already have selected the highest point you can accommodate for this antenna in as open a setting as possible.

  • Be mindful of the Near Field. Near field affects anything metallic within a half wave distance to the proposed antenna installation point. As a rule of thumb I assume 50 foot sphere surrounding my antenna is near field for my operations which include mostly 40 meters and above. Since this is my number one concern, how do I deal with it? I cant get my stealthy and CC&R proof antenna 50 feet away from everything on my city lot? Well, simply put, I compromise. My antenna is 27 feet above ground but does in fact overhang my garage area. This means that my radiator will be tightly coupled to any and all wiring and metal objects within that 50 foot sphere. I know from this that I will have to choke off all my house wiring within that 50 foot sphere. To that end, I ferrite my romex wiring and other cables such as cable tv wiring and telephone wiring within that range. Yes. It is a pain in the Tuckus but it saves heartbreak later when you cant operate while your wife and kids use the appliances.
  • Use horizontal instead of vertical polarization. Even though it may be harder to accomplish, the results are worth it. In practical installations (simple dipole etc) there is much less worry about lossy earths and horizontal antennas get an extra kick from far field reflection (adding to signals) which verticals do not.
  • Skip this paragraph if you only talk to S-9 signals. However, if you are like me and want to talk to stations in rare countries with weak signals it is worth your time to study common mode choking. Choke the feed line multiple times along its route. There are a number of good papers on the Internet describing why to do this but suffice to say it is the single best way to reduce received noise levels I have ever experienced. I have links in my Blogroll pointing to a particularly good white paper on choking. Since chokes are bilateral they keep RFI out of your shack as well. Don’t be disillusioned by some papers you see claiming this can only be done with coax cable feed lines. Choked balanced lines work very well too. Test your received noise level by opening up your bandwidth to as wide as your receiver will allow and use AM Mode for receive. What does your S-Meter read? S-9? More? Mine began at S-9 plus 20 DB or more and now reads S-5-6. Choking is worth the effort.
  • Learn where your losses are coming from. Feed line losses are taken for granted in cases such as using balanced feed lines. Don’t take them for granted. Check some of the links in my Blogroll. In my case I rebuilt my feed line to allow for transmitting 500 watts and supporting some fairly substantial choke kit. My feed line is heavy duty-made from one eighth inch phosphor bronze wire rope. Not really necessary but I found the material surplus so I used it. You can just as easily use some number 10 house wiring or some surplus from ebay or one of the surplus stores around. I used this heavy material to support my heavy duty chokes as well as reduce ohmic loss. From my dipole and EZNEC 4 I learned that replacing the 22 gauge wire with 10 gauge wire for the antenna saved about 6% of lost power. Choking other conductors within the near field tends to reduce losses from my antenna because less power is sapped up by extraneous non-useful radiating conductors. I don’t know how to model this but it stands to reason that E and H fields exciting local non antenna radiators sap up energy from the source antenna. If it isn’t going to the ionosphere it isn’t helping and quite literally could be hurting by creating havoc within your homes appliances. If, like me you use a tuner, make sure it is a good one with good quality parts. It is very easy to buy an inexpensive tuner with inferior parts only to find out that you are dissipating several hundred watts in the tuner components before they even get a chance to go to the antenna. QST has published some good efficiency reviews on commercial tuners. Learning where your losses are is extremely important for low power stations. Yet, I have never seen folks addressing these issues in QRP circles which is surprising as that group tends to be on the forefront of pushing the envelope with HF experimenting.
  • Choke all interior wiring including (especially) broadband modems, solid state relays, low voltage lamps and other innocuous noise sources. Use a portable radio to locate the bad actors. My worst offender was Direct TV’s HDDVR unit. Choke both power and signal lines. This is very important when considering noise abatement in your local environment.

A couple of final thoughts. RF is a lot like insects: if you can see or hear the symptoms of a signal it is going to be everywhere. You may get tired of chasing it but it sure isn’t boring. Second, you saw nowhere in my discussion anything about SWR. A properly excited non resonant dipole fed with an efficient feed line and a good tuner is a potent weapon. I am a refugee of ham stations with 70 foot towers and big antennas. I have never had more fun doing radio than I have in the past two years with this simple dipole setup. There is great satisfaction in being able to operate within the confines of a suburban lot, make a big noise on HF and not bother anyone else in or around my home.

Finally, let me tie things up with a rationale for paying attention to the installation criteria as I have outlined. From a city lot and no tower or gain antenna, to compete in DX’ing or make some noise in a contest or two, you need to hear things before the spotting cluster-bums do. To actually discover the elusive prey before others is challenge enough, but to work them you need to discover them and then beat everyone to the punch by being early in any pile ups. You are working at a disadvantage, however, guile and craftiness can mitigate the application of all the usual recipes for success in DX’ing, Contesting or just enjoying ham radio-that being large towers and big arrays high in the sky.

Thanks for reading. Best, Chas W7MAP/5

Advertisements
 
Leave a comment

Posted by on November 25, 2008 in Radio

 

Tags:

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

 
%d bloggers like this: