antenna booms
The following discussion about the advantages/disadvantages of grounding antenna booms took place in the Moon-Net reflector. I found the subject so interesting that I decided to put together all the messages in this page.
On 12-Feb-2005 VK3FMD wrote:
What are the advantages/ disadvantages of grounding antenna
booms?
I have insulated element design and have fabricated 4:1 1/2 Lambda baluns with folded dipole feeds. I am trying to decide whether or not to connect the braid to the booms at the feed point. Previously I have done this with other antennae , however it was more by mechanical design than a concious decision to do this. I am not sure of the reasons! I see most commercial designs that I have come across do in fact connect the braid to the boom.....I suspect it would help reduce static build up on the boom...but are there other reasons?
All comments appreciated.
On 12-Feb-2005 SM5BSZ wrote:
For the desired signals it should not matter at all.
For undesired signals, 88 to 100 MHz al well as for much lower frequencies the difference may be very large.
I suggest that you connect the braids to the boom tube AND that you connect the midpoint of your folded dipole to the braids too.
(to avoid changing the electrical length by having the midpoint in contact with the boom tube at both sides you can use wires connected to the element several mm away from the boom. One wire on each side to retain symmetry. Connect to the braids and the boom tube in some symmetrical fashion.)
A folded dipole with the midpoint connected to the balun braids is a good filter (with absolutely no losses for the desired signal.) This filter will reduce the risk that you run into problems with intermodulation from out-of-band signals.
The peak power reaching your pre-amplifier during thunder-storms will be dramatically reduced. I always make sure to have this protection. It is "free" on a folded dipole. With open dipoles it is done by adding a shorted 0.25 wl stub.
Another thing, it may be a good idea to ground the braid to the tower at the point where the cable leaves the tower and again ground the braid at a good point where it enters your schack.
To get the idea: Imagine someone put a toroid around the cable between your house and the tower and used the toroid to inject 1000A of RF current anywhere between 100kHz and 30 MHz. The braid should be grounded in such a way that your rig would not become destroyed. (Lightening protection)
You could use a signal generator and inject a few mA. Then measure currents and voltages in the schack:-)
Having all cables from the tower in a tight bounch and as many as possible screened and all screens and ground wires grounded like the main coaxial cable helps to make the installation survive thunder-storms.
On 12-Feb-2005 G3LTF wrote:
Hi Leif and Charlie, when you ground the centre of the
folded dipole to one side of the boom and the braids to the other you do have to
be very careful not to introduce a dissimilar metals problem which will...and it
really happened to me last year....give extra noise pick up. I used stainless
steel self tapping screws and washers for the connection to the boom.. I should
add that this is a totally HB 4.5L Yagi, a mix of DL6WU and DJ9BV ideas.
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