Attempt to insert value via "attr"

Question

I’m creating a page that will simulate a website in the mobile version.

After filling in the URL field, the site will appear in an "Iframe".

The problem is that when I enter the URL into the field, some sites work and others don’t. I am passing the value of the field via "attr". Has anyone ever been through this or would you know how to help me? Look at the code:

function simula(){
  var x = document.getElementById('campo1').value;
  $('iframe').attr('src', x);
}

#campo1{width: 300px; padding: 10px;}
.bt{padding: 10px 40px; background: #000; color: #fff; border: 0px; cursor: pointer;}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<p align="center">
     <input type="text" name="campo1" id="campo1" placeholder="Inserir URL"><br><br>
     <button class="bt" onclick="simula()">TESTAR</button>
<br><br><br>
     <iframe name="Iframe" class="frame"  width="800" height="400"></iframe>
    </p>