Low Dipoles and Zepps - Considerations

Submitted by Tom K1JJ


From a Bulletin Board Posting:

 Tom, What is your opinion of a flat top 162' each side of center feed with open wire line to a balanced tuner? The feed point is aprox 50' and one leg is about 30' from the ground and the other about 10'. The feed line is about 80' to the tuner. By the way, the mod monitor is working great.
: 73,Bob

Hi Bob,

It looks like you're trying to cover 75M AND 160M too.

Well here's a few things to think about:

The closer to the ground a horizontal antenna is the lower it's efficiency due to ground coupling - losses. If you model a dipole at 30' high and one at 60' you will see that the patterns are kinda similar BUT the lower one's pattern is smaller, shrunken - losses in the ground.

Yes, you get a good cloud burner at 90 degrees straight up, but at a steep price paid for losses and lower angle contacts.

Now, I've found that when comparing two-half waves in phase against a 1/2 wave dipole, the two half waves need to be up about 20' higher to achieve the same vertical pattern as the 1/2 wave dipole. IE, the longer the antenna, the more ground coupling takes place.

I've noticed this effect when making large 75M yagi arrays and found that the simple 1/2 wave dipole elements worked better at 120' than the longer ones.... lower vertical takeoff angle.

If you have unlimited height to work with, that's a different matter, but if you are limited to a height of 45' or so, a 1/2 wave dipole on 75M will have a lower takeoff angle than an extended zepp at the same height, etc.

So, if height is a problem, then use a separate dipole for 75M and then do the separate one for 160M.

BTW, an antenna's EFFECTIVE height is usually determined by it's average height. So in your example, if the apex is at 50' and one leg is at 30' and the other leg is at 10', we may have an effective height of maybe only 33' high or so. This antenna will have unacceptable ground losses on 160M.

But on 75M you will do OK - Though a 1/2 wave 75M dipole would have it's legs higher in that same situation due to its shorter length and have less ground coupling too- superior.

BTW, Steve, HUZ sent me some new data on the "brown crap" open wire. It seems you don't wanna use that stuff in the rain with SWR... unbelievable. Use good quality homemade openwire. Better yet, in your situation, I would use two separate dipoles, each fed with coax - They will not interact enough to notice the difference even if right near each other.

Just one ham's opinion... :-)

Tom, JJ