Posts Tagged ‘antennas’
Watch the video
Then get the detailed instructions and templates
Check out the U.S. Marine Corps Field Antenna Handbook- 192 pages
Ok, lets see if we can size up the situation.
It’s June 12, 2009 and there is no longer any analog television starting today…
… and your digital television reception is really bad…
… and you want a weekend project…
… and you like building antennas…
Well, we have a solution for you.
How about a weekend project to build your own HDTV Digital TV antenna ??
Kent Britain, WA5VJB, has great weekend construction project for you.
Kent writes antenna columns for CQ, CQVHF, and Popular Communication plus a microwave column for the German magazine DUBUS.
So. there you go – how much better can it get than that?
Small Transmitting Loop Tuners from MFJ
The biggest challenge in building the small transmitting loop is not the wire loop – it’s getting the tuning capacitor. The images I posted in the AEA Isoloop article showing homebrew loops showed that most of the capacitors used in those homebrew loops were “ham fest specials” or from the junk box.
MFJ Enterprises solves the problem of getting the capacitor for your loop antenna. They have an number of loop tuners from $160 to $260.
The $160 model covers 160m to 10m and handles 50 watts. The $260 model which also covers 160m to 10m can handle 300 Watts and has a built-in SWR Meter as well as loop current meter. I don’t know what significant parts are inside the $160 model other than the tuning capacitor but $160 seems like a lot of money for a box with a capacitor and a few other parts and no tuning indicator (loop current meter).
I can’t remember exactly how much I paid for the AES Isoloop. It was about $329. So. for another $70 or so what I got in addition to the (expensive) tuning capacitor is the stepper motor which allows remote tuning. Why remote tuning? Watch out for the RF!
RF Exposure in Loop Tuning
My basement has a collection of limited space antennas. They don’t work well in the basement. Oh yes, I forgot, they are there in the basement in storage. All these antennas have a story to tell. They were used, at one time or another, in one of my limited-space or stealth-operating living locations.
The AEA IsoLoop HF Antenna was an antenna I used off the balcony in a Chicago high-rise. The IsoLoop was a little more stealthy than my Texas Bug Catcher with its 10ft length at about a 45 degree angle hanging over the edge of the balcony within sight of my neighbors.
The AEA IsoLoop is a small loop transmitting antenna that covers 14 Mhz to 30 Mhz continuously tunable. Being continuously tunable is a good thing and its a bad thing. It’s a good thing as one antenna can go any where between 20 meters and 10 meters – anywhere. The bad thing is that it’s tunable and can go any where between 20 meters and 10 meters. Got that? It’s tunable – which means you have to make some sort of adjustment for each band on which it operates.. Unlike a multi-band antenna that is resonant on multiple bands at the same time, a small loop antenna, like the screwdriver antenna is resonant on one swath of frequencies at a time.
The veteran – The Trusty Texas Bug Catcher
I am not sure how it got it’s name, but if you have a Texas Bug Catcher antenna mounted mobile you are bound to pick up a few – bugs that is. If you have an air coil then cleaning those bugs out of there could be a chore.
Much safer is to mount the Bug Catcher in a stationary location. Yes, it is possible and I have gotten good results. Perhaps it’s overlooked, but a Texas Bug Catcher makes a good portable antenna, limited-space antenna, and a good stealth antenna for those communities where antennas are considered an eyesore.
I been using a Bug catcher for more than a decade. I first used it on a balcony bolted to a steel railing in a Chicago high rise on the 40’th floor – 400 ft up. When I moved to a location on the ground I used it in the backyard and on camping trips.
For portable or backyard use all you need is an 18 inch galvanizedpipedriven into the ground with a sledge hammer. Mount the Bug Catcher to the pipe using common mounting hardware you can find at a truck stop or Radio Shack that sells CB mounting hardware (3/8-24 stud)
The Catch to the Bug Catcher
If you take a look at the Bug Catcher you can see the pain points of this antenna – it has taps. Yes, it’s “continuously tuneable”, you can set the taps any place you want. But after you have the tap points set – that’s it – those are the taps points you use until you change them.
Pain Points = Tap Points
First, the tap points will be the only ones you can use until you change them. Second, finding the tap points can be painful. Heaven help you if you don’t have an antenna analyzer. It is going to be a long process of set-and-test until you find the tap points of the center points of the band portions that you want to work.
For those folks who have constraints that do not allow you to put up a big HF antenna then take a look at the MFJ-1796 6-band antenna. This antenna covers 5 HF bands and the 2 meter band. HF bands covered are 40m, 20m, 15m, 10m, and 6m.
We’ve been using this antenna for about 8 years on HF with good results. To have one antenna cover all these bands is a convenience.
At the outset we should say that this review of the MFJ-1796 is for a very special circumstance. We have made a slight modification to the antenna and we use it in a slightly different position than recommended.
Look at the picture to the left. That is the antenna that MFJ will deliver. But we have changed it a bit – read on.
The word from MFJ
The quote below is directly from the MFJ-1796 assembly manual and briefly describes the antenna structurally and electrically.
The basic 40 meter quarter wave vertical antenna is 33′ tall and requires a reasonably good ground or counterpoise system to function properly. The usual way to eliminate the requirement for a complicated and space consuming ground system is to center feed a 1/2 wave (in this example a 66′ tall) antenna.
The six and two meter amateur bands are covered with the addition of four quarter wave decoupling stubs. The power rating of the antenna is 750 watts on six meters and 300 watts on two meters.
MFJ solved these problems by combining efficient end loading with a balanced center feedpoint design. The result is a physically small vertical antenna that gives good performance and does not require any type of RF ground system.
The reduction in size is accomplished by adding separate loading coils and capacitance hats at each end of the antenna for the HF bands. The efficient end loading coils are wound on fiberglass forms. The high quality materials and construction of the HF loading system allows a maximum power rating of 1500 watts on 40, 20, 15 and 10 meters. The continuous CW power ratings are 500 watts on 40 meters, 750 watts on 20 or 10 meters and 1000 watts on 15 meters.
So lets get to the bottom line on this antenna – its a balanced dipole with loading coils (not traps) and capacity hats at each end. The length of the antenna is 12 feet as delivered from MFJ. Its as simple as that.
The MFJ-1796 as delivered by MFJ is expected to be used as a vertical and mounted on the ground, on a tripod on a roof, or attached to a chimney. Its a center-fed vertical dipole that is ground independent – that is, it does not require a counterpoise or grounding system.