



		You need a Java enabled browser!

Antennas: Date (4-19-1999)

I have been in the process of either buying or building a new antenna. I purchased a "gap" antenna that did not perform well for me at all. With no balun in the feed to the antenna, I had very bad RFI. On 80 meters It had the performance of a dummy load. I mounted the antenna about 35ft high on my tower. The company worked with me to solve the problem for 2 weeks with no luck (I did all they ask) and they charged me a 15% restocking fee to return the antenna. Now, I am back to finding a new antenna.

The new antenna must have all bands 80-10 meters, and fit into no more than 65 ft. With these restrictions there are four ways to proceed:

	MFJ 1798 Vertical.
	Build a Spider Dipole with loading coils for 80,
	B & W folded Dipole,
	Cushcraft R7000

MFJ 1798 Vertical.

From MFJ Home page:

" Operate 10 bands -- 75/80, 40, 30, 20, 17, 15, 12, 10, 6 and 2 Meters -- with this MFJ-1798 vertical antenna and get full size performance with no ground or radials!

Full size performance gives you high efficiency for more power radiated. The result? Stronger signals and more Q-5 QSOs.

Full size performance also gives you exceptionally wide bandwidths so you can use more of your hard-earned frequencies.

You get very low radiation angle for exciting DX, fully automatic bandswitching, omni-directional coverage, low SWR and full 1500 watts PEP SSB power handling.

Full size performance is achieved by using separate full size radiators for 2 through 20 Meters and highly efficient end loading for 30, 40 and 75/80 Meters. You get highest possible efficiency and exceptionally wide bandwidths.

MFJ's unique Elevated Top Feed™ puts the maximum radiation point high up in the clear where it does the most good -- your signal gets out even if you're ground mounted.

Self supporting and just 20 feet tall, the MFJ-1798 mounts easily from ground level to tower top -- on small lots, backyards, apartments, condos, roof tops, tower mounts.

Separate Full Size Radiators
Separate full size quarter wave radiators are used on 20, 17, 15, 12, 10 and 2 Meters. On 6 Meters, the 17 Meter radiator becomes a 3/4 wave gain radiator.

The active radiator works as a stub to decouple everything beyond it. In phase antenna current flows in all parallel radiators.

This forms a very large equivalent radiator and gives you incredibly wide bandwidths.

These radiator stubs provide automatic bandswitching -- there is absolutely no los due to loading coils or traps.

No Feedline Radiation
The feedline is decoupled and isolated from the antenna with MFJ's exclusive AirCore™ high power current balun. This highly efficient balun is wound with Teflon® coax. It can't saturate, no matter how high your power.

This important decoupling attenuates RF on the outside of your coax and minimizes feedline radiation that wastes useful power and distorts your pattern."

This antenna is top loaded vertical. Top loading means better field strength. (performance)

Very good bandwidth:

The following chart lists the power rating and the 2:1 VSWR bandwidth of this antenna:

Band	Power			Bandwidth
	CW	SSB	RTTY
80	1000	1500	750	35 MHz
40	1250	1500	1000	25 MHz
30	1500	1500	1500	370 MHz
20	1500	2500	1500	700 MHz
17	1500	2500	1500	680 MHz
15	1500	2500	1500	800 MHz
12	1500	2500	1250	750 MHz
10	1500	2500	1250	2.0 MHz
6	300	750	30	2.4 MHz
2	200	200	200	7 MHz

The draw backs of this antenna is that it is vertically polarized. The antenna has a low angle of radiation. Translated, this means DX can hear you better than local stations. Therefore, close 80 meter contacts are less likely to hear you well. You only have 1/4 wave on 40 and 80 meters, remember the more antenna surface area the better your receive!

Good over all Antenna.

Cost about $ 269.00

ORDERING: Order any product from MFJ and try it for 30 days. If you're not delighted, they will refund your purchase price, less shipping charges

For more information on verticals look at the vertical antenna section here

For information on the MFJ 1798 look here

The 80, 40, 20, 15, 10 meter Spider Dipole with loading coils for 80.

Antenna shortening and loading: Although antenna loading for the sake of shortening reduces the feedpoint impedance, it introduces components that raise antenna Q and narrow the bandwidth. As a rule of thumb, bandwidth is reduced by the percentage of shortening of the antenna. Measured SWR curves for loaded multi-band dipoles are here.

For more information look at the loaded dipole section here

also look at Coils, Linear Loads, and Capacity Hats:
An Overview of Small Loaded Yagis

You can calculate the space required and coil needed for this antenna at ham Calc.

Drawbacks of this antenna are: narrow bandwidth on 80 meters, hard to tune on 80 meters. All other bands are atypical 1/2 wave dipoles.

Cost about $ 50.00

Built cost $ 150.00

For information on the spider dipole look here

B & W folded Dipole

From B & W Web Page:

"This unique antenna design covers a continuous frequency range up to 1.8 to 30 MHz, with a VSWR of less than 2:1 across the entire range when fed with a 50 ohm coaxial cable. No antenna tuner is required. The fact that this antenna is folded over on itself gives the advantage of EMI rejection far greater than that of a random wire or a conventional dipole, making receive mode extra quiet. There are no adjustments required when changing frequency, making operation simple for untrained commercial personnel, amateurs, and professionals alike. There are no mechanical parts like relays or motors to wear out, as in some other broadband tuning systems!

It's Simple . . . It's Reliable . . . It Works!

These antennas are ruggedly built. We only use stranded copperclad steel, and stranded stainless steel wire. Many have withstood hurricanes, making them an ideal choice for emergency services and mission critical installations. Tens of thousands are in service on every continent in the world, from the tropics to the antarctic regions. Installation is simple, allowing configuration as an inverted Vee, flat top, or sloper. Recommended minimum height above ground is 25 feet (7.6 M). The higher the installation, the more effectively the antenna will radiate, especially at the lower frequencies."

This antenna is essentially a small non-resonant rhombic traveling wave antenna. It has 4 sides and a terminating resistor. The rhombic antenna is usually described as the "King of antennas" because of it's very desirable characteristics. Rhombics are a unidirectional antenna with broad bandwidth.

Look here for more information on Rhombic Antennas

Drawback of this antenna is the terminating resistor, The terminating resistor and ground effect uses about a 1/3 of the input power. (according to The ARRL Antenna Book). Limited output to 1KW pep. Also if you want 80 meters the antenna is 90 ft. The 5mhz - 30 MHz is 65ft long.

Ordering: I emailed Jeffrey Engel at B & W, he would not commit to a return policy with his antenna.

Cost about $ 198.00

For information on the Folded Dipole look here

Cushcraft R7000

Frequency, meters       10,12,15,17,20,30,40
Gain, dBi               3
Wavelength each band    Half-wave
VSWR                    1.2:1 typical
2:1 bandwidth, kHz      10m (1700), 12m (100)
                        15m (450), 17m (100)
                        20m (250), 30m (100)
                        40m (150)
Power Rating, Watts     1500
Radiation angle, deg.   16
Horizontal rad, deg     360
Height, ft(m)           24 (7.3)
Mast size range, in(cm) 1-3/4 to 2-1/8 (4.4-5.4)
Wind load, sq ft (sq m) 2 (0.2)
Weight, lb, (kg)        18 (8.2)

RELIABLE
New trap design is stable in all conditions.

EASY INSTALLATION
For typical use, tuning is not needed after installation.

AUTOMATIC BAND CHANGING
to any band from 10 through 40 meters (80 m with R80 kit)

SLIM SILHOUETTE
Gain favor of family and neighbors with the slim, smooth profile of our new trap design.

BASE ASSEMBLY

The R7000 is an expandable, seven band vertical antenna. The radiator is electrically a halfwave length on each band. The six traps control the electrical length. Your signal is applied at the base of the antenna by using a broadband matching network. Since impedance at the base is very high, a 4:1 broadband bifilar transformer is used. A coaxial balun is used to keep RF from migrating down the coaxial line.

Look here for more expiation.

THE BLACK BOX REVEALED

The draw backs of this antenna is that it is vertically polarized. The antenna has a low angle of radiation. Translated, this means DX can hear you better than local stations. This antenna unlike the MFJ 1798 has no loading coils, but traps giving it very narrow bandwidth. But it has more physical antenna length, yields stronger receive signals.

After considering all of the above, I have decided to use the MFJ 1798. Because of the wide bandwidth (largest of all the antennas above) gain and overall performance of the antenna.

I also was very impressed with MFJ's return policy's and interest in working with there customers.

Update 5-19-2000

Some problems accrued over the winter months with the antenna. You can read all about it here.

So, I have finally decided to go with a proven performer and manufacture. The A3S w/40 meter add on kit by Cushcraft. Installation of the antenna is scheduled for June 3, 2000. I will be posting pictures of the antenna assembly and raising here.