VHF 1/2 wave vs. 5/8 wave in attic install

Status
Not open for further replies.

timkilbride

Member
Database Admin
Joined
Feb 9, 2006
Messages
1,809
Location
Iowa County, Iowa
Currently I have a NMO mobile to base kit with a VHF 5/8 wave antenna on it. I have drilled a hole in the roof and have the aerial part of the antenna above the roof. The coil, feed point, and ground plane rods are below the roof.

My question is, would I just be better off using a 1/2 wave so I don't have a ground plane to deal with? I don't have a 1/2 laying around to experiment with. Looking for comments on this. I attached a picture of the current setup.

Tim K.
 

Attachments

kb2vxa

Completely Banned for the Greater Good
Banned
Joined
Mar 22, 2005
Messages
6,131
Location
Point Pleasant Beach, N.J.
Unless you're thinking about a dipole you still need a ground plane for a 1/2 wave antenna. Now why did you go through all the trouble of drilling a hole in the roof only to make a mess of it and end up with a detuned antenna? Now you can think about shoving the coax up through the hole and putting the antenna outside or putting the whole shebang in the attic and plugging the hole.
 

timkilbride

Member
Database Admin
Joined
Feb 9, 2006
Messages
1,809
Location
Iowa County, Iowa
Unless you're thinking about a dipole you still need a ground plane for a 1/2 wave antenna. Now why did you go through all the trouble of drilling a hole in the roof only to make a mess of it and end up with a detuned antenna? Now you can think about shoving the coax up through the hole and putting the antenna outside or putting the whole shebang in the attic and plugging the hole.
Yes, I was thinking about a dipole. I live in an apartment so I have no access to the roof. I do have a folded dipole in the attic, but the 5/8 wave through the roof is outperforming it. I was looking for a way to get rid of the ground and I thought(and was wrong) a 1/2 didn't require a ground.

Tim K.
 

timkilbride

Member
Database Admin
Joined
Feb 9, 2006
Messages
1,809
Location
Iowa County, Iowa
Without access to the roof, you have no way to seal the external exit hole around the antenna. You can seal the inside, but water will wick down the outside portion of the antenna and go into the substructure of the roof.
Its actually a rubber cap deal I made the hole in.

Tim K.
 

Cochran_rick

Member
Joined
Feb 10, 2005
Messages
23
Location
Prattville, Alabama
Your 5/8 wave has a lower angle of radiation then your 1/2 wave so you should notice an improvement there, but as the other post said, you still need a ground plane. How high is your dipole off the ground? If it isn't at least a 1/2 wave length off the ground it will be radiating at a high angle and you won't get it's full potential. Good luck if your landlord finds out about your "hole."
 

prcguy

Member
Joined
Jun 30, 2006
Messages
9,763
Location
So Cal - Richardson, TX - Tewksbury, MA
Generally speaking, a 1/2 wave end fed antenna does not require a ground plane and it radiates the same as a center fed 1/2 wave or dipole. If you have a store bought 1/2 wave mobile antenna it will probably have a little trouble poking through the roof as you described because the end near the matching coil is very high impedance and anything touching it will detune the antenna.

You could use a 1/2 wave center fed dipole but half the antenna would be inside the attic.

A 5/8 wave antenna poking through the roof will be at a much lower impedance where the roof penetration touches the antenna and will have less of a problem.
prcguy


Unless you're thinking about a dipole you still need a ground plane for a 1/2 wave antenna. Now why did you go through all the trouble of drilling a hole in the roof only to make a mess of it and end up with a detuned antenna? Now you can think about shoving the coax up through the hole and putting the antenna outside or putting the whole shebang in the attic and plugging the hole.
 

n5ims

Member
Joined
Jul 25, 2004
Messages
3,804
Hopefully the antenna is noticed and removed prior to somebody falling off the roof by not seeing it first and tripping over it. This would cause some serious liability issues for you well beyond just being evicted. Especially if that worker is seriously injured or worse yet, killed.

Stupid acts like this are what makes apartment owners and their management have one, and only one, answer to questions about installing antennas. That answer is NO. While I believe that you didn't intend any harm by doing what you did (only wanting to not have it noticed), it probably wasn't the best decision to mount it that way.
 

kb2vxa

Completely Banned for the Greater Good
Banned
Joined
Mar 22, 2005
Messages
6,131
Location
Point Pleasant Beach, N.J.
Looks like John and prcguy caught on, now is time for four things.
1) Drop the whole antenna into the attic and hope for the best.
2) Ask yourself how the heck to plug the hole so water can't get under the shingles and rot the roof.
3) Ask yourself why you drilled a hole in the first place.
4) Kick yourself.
 
Status
Not open for further replies.
Top